Geochemical Processes at Mineral Surfaces 9780841210042, 9780841211605, 0-8412-1004-7

Table of contents :
Title Page......Page 1
Half Title Page......Page 3
Copyright......Page 4
ACS Symposium Series......Page 5
FOREWORD......Page 6
PdftkEmptyString......Page 0
PREFACE......Page 7
1 Geochemical Processes at Mineral Surfaces: An Overview......Page 9
Physical Properties of the Mineral-Water Interface......Page 10
Adsorption......Page 12
Ion Exchange......Page 14
Surface Spectroscopy......Page 16
Dissolution, Precipitation, and Solid Solution Formation......Page 17
Transformation Reactions at the Mineral/Water Interface......Page 20
Literature Cited......Page 21
2 Simulating Liquid Water near Mineral Surfaces: Current Methods and Limitations......Page 26
General Simulation Approaches......Page 27
Interaction Potentials......Page 29
Simulations of Interfacial Water......Page 34
Summary......Page 38
Literature Cited......Page 40
3 Behavior of Water on the Surface of Kaolin Minerals......Page 43
Clay Mineral Structures......Page 44
Experimental Studies of Water on Kaolin Minerals......Page 49
Application of Results to other Silicate Minerals......Page 56
Summary......Page 57
Literature Cited......Page 58
4 Reactions at the Oxide-Solution Interface: Chemical and Electrostatic Models......Page 60
The Oxide-Electrolyte Interface in Geochemistry......Page 61
Conceptual Model of an Adsorption Reaction......Page 63
Surface Complexation Models......Page 65
Electrostatic Models for the Electric Double Layer......Page 70
Interpretation of Data......Page 74
Concluding Remarks......Page 81
Legend of Symbols......Page 82
Literature Cited......Page 83
5 Surface Potential-pH Characteristics in the Theory of the Oxide-Electrolyte Interface......Page 85
Model for an Amphoteric Surface without Complexation......Page 86
Model for an Amphoteric Surface with Complexation......Page 92
Determination of Surface Equilibrium Constants......Page 95
Conclusions......Page 100
Legend of Symbols......Page 102
Literature Cited......Page 103
6 Free Energies of Electrical Double Layers at the Oxide-Solution Interface......Page 105
a . The Oxide Surface......Page 106
b . The Diffuse Double Layer......Page 108
c . The Equilibrium Point......Page 109
Free Energy of Formation of One Surface......Page 112
Interaction Free Energy Between Two Plates......Page 115
Literature Cited......Page 118
7 Mechanism of Lead Ion Adsorption at the Goethite-Water Interface......Page 119
Experimental Materials and Methods......Page 120
Equilibrium Model......Page 122
Triple Layer Model......Page 123
Equilibrium Results......Page 126
Kinetic Model......Page 130
Kinetic Results......Page 132
Summary......Page 137
Pressure-Jump Apparatus with Conductivity Detection......Page 138
Net Proton Release and Ligand Number......Page 141
Derivation of the Time Constant Relationship of Equation 35......Page 143
Legend of Symbols......Page 144
Literature Cited......Page 145
8 Characterization of Anion Binding on Goethite Using Titration Calorimetry and Cylindrical Internal Reflection-Fourier Transform Infrared Spectroscopy......Page 147
Titration Calorimetry......Page 148
Cylindrical Internal Reflection - Fourier Transform Infrared (CIR-FTIR) Spectroscopy......Page 155
Acknowledgments......Page 163
Literature Cited......Page 164
9 Macroscopic Partitioning Coefficients for Metal Ion Adsorption Proton Stoichiometry at Variable pH and Adsorption Density......Page 167
Determination of χ(ρΗ,Γ) as a Macroscopic Model Parameter......Page 174
Summary......Page 191
APPENDIX I THE MATHEMATICAL DEVELOPMENT OF ISOTHERM ANALYSIS FOR THE MACROSCOPIC PROTON COEFFICIENT, Xp......Page 192
Literature Cited......Page 194
10 Sorption of Hydrophobic Organic Compounds by Sediments......Page 196
Adsorption Mechanisms......Page 197
Partitioning Thermodynamics......Page 199
Nonidealities in Humic Polymers......Page 202
Aqueous-Phase Nonidealities......Page 205
Limitations of Partitioning Theory......Page 208
Sorption Kinetics......Page 213
Summary and Conclusions......Page 216
Symbols:......Page 217
Acknowledgments......Page 218
Literature Cited......Page 219
11 Distinguishing Adsorption from Surface Precipitation......Page 222
Solubility Methods......Page 223
Kinetics Methods......Page 227
Surface Spectroscopy......Page 229
Literature Cited......Page 231
12 Adsorption-Desorption Kinetics at the Metal-Oxide-Solution Interface Studied by Relaxation Methods......Page 234
Experimental Methods and Materials......Page 235
Surface Reaction Kinetics......Page 236
Intercalation Kinetics......Page 248
Literature Cited......Page 256
13 Highly Selective Ion Exchange in Clay Minerals and Zeolites......Page 258
Thermodynamic background......Page 259
Influence of interlayer charge density and interlayer hydration......Page 260
Exchange of cations of high polarizability......Page 269
Highly selective exchange in illite and modified montmorillonites......Page 278
Reversibility......Page 284
Highly selective exchange as related to the presence of crystallographically different site groups......Page 287
Exchange of transition metal ion uncharged ligand complexes......Page 292
Literature Cited......Page 294
14 Potassium Fixation in Smectite by Wetting and Drying......Page 300
Previous Work......Page 301
Aims of the Present Study......Page 302
Materials and Methods......Page 303
Experimental Results......Page 307
Conclusions......Page 326
Literature Cited......Page 328
15 Potassium-Calcium Exchange Equilibria in Aluminosilicate Minerals and Soils......Page 331
Methods......Page 332
Results and Discussion......Page 334
Conclusions......Page 342
Literature Cited......Page 343
16 Adsorption of Metal Ions and Complexes on Aluminosilicate Minerals......Page 345
The nature of clay surfaces......Page 347
Adsorption of metal ions on minerals......Page 349
Characterization of chemical forms of adsorbed metal ions......Page 351
Adsorption and characterization of complex species on minerals......Page 356
Reactions involving metal ions adsorbed on minerals......Page 358
Conclusions......Page 360
Literature Cited......Page 361
17 Paramagnetic Probes of Layer Silicate Surfaces......Page 365
Materials and Methods......Page 368
Results and Discussion......Page 369
Literature Cited......Page 390
18 Applications of Surface Techniques to Chemical Bonding Studies of Minerals......Page 392
X-Ray Photoelectron Spectroscopy (XPS)......Page 394
Auger Electron Spectroscopy (AES)......Page 398
Combined X-Ray Photoelectron/X-Ray-Induced Auger Spectroscopy(XPS/XAES)......Page 400
Literature Cited......Page 404
Principles......Page 406
Experimental Section......Page 408
Experimental Results......Page 409
Analysis of the Mossbauer Data......Page 417
Discussion......Page 422
Future Prospect......Page 426
Literature Cited......Page 427
20 Photoredox Chemistry of Colloidal Metal Oxides......Page 428
Light Absorption by the Metal Oxide Bulk......Page 429
Light Absorption by Surface-Located Chromophores Incorporating the Metal of the Oxice Lattice......Page 431
Effect of Metal Oxide Surface Properties on Photoinduced Redox Reactions......Page 443
Literature Cited......Page 445
21 Adsorption of Organic Reductants and Subsequent Electron Transfer on Metal Oxide Surfaces......Page 448
Reductive Dissolution: Overall Reaction Scheme......Page 449
Surface Chemical Reaction Mechanism......Page 450
Description of Reductive Dissolution on a Molecular Level......Page 453
Conclusions......Page 461
Literature Cited......Page 462
22 Abiotic Organic Reactions at Mineral Surfaces......Page 464
Reactions Promoted by the Lewis and Bronsted Acidity of Clay Minerals......Page 466
Oxidation of Organic Compounds by Oxides and Primary Minerals......Page 482
Acknowledgments......Page 485
Literature Cited......Page 486
23 Mn(II) Oxidation in the Presence of Lepidocrocite: The Influence of Other Ions......Page 489
Objectives......Page 490
Experimental......Page 491
Results and Discussion......Page 492
Mn(II) Oxidation in Natural Waters: Implications of Experimental Studies......Page 499
Acknowledgments......Page 502
Literature Cited......Page 503
24 Interaction of Co(II) and Co(III) Complexes on Synthetic Birnessite: Surface Characterization......Page 505
Methods and Materials......Page 507
XPS Studies/Characterization......Page 509
SIMS Analysis......Page 516
Summary......Page 521
Literature Cited......Page 523
25 Ionic Solid Solutions in Contact with Aqueous Solutions......Page 525
The Roozeboom Classification......Page 526
DISTRIBUTION LAWS AND SUBSTITUTIONAL DISORDER......Page 534
Application In Analytical And Inorganic Chemistry......Page 536
The System CaCO3 - MqCO3......Page 541
The System Hydroxyapatite - Fluorapatite......Page 545
Literature Cited......Page 559
26 Approach to Equilibrium in Solid Solution-Aqueous Solution Systems: The KCl-KBr-H2O System at 25°C......Page 562
KCl-KBr-H2O System at 25°C......Page 563
Theory......Page 564
Equilibrium Constants......Page 567
Test for Equilibrium......Page 568
Conclusion......Page 573
Literature Cited......Page 574
27 Modes of Coprecipitation of Ba2+ and Sr2+ with Calcite......Page 575
The Incorporation of Sr(II) into Calcite: A Review......Page 576
Experimental Methods for Coprecipitation of Ba(II) with Calcite......Page 577
Ba(II) Experiments:Results and Discussion......Page 578
EPR Studies of Sr(II) and Ba(II) in Calcite......Page 584
Implications......Page 585
Literature Cited......Page 586
Experimental Results......Page 588
Experimental Results......Page 589
Calculation of Diffusion Parameters......Page 593
Discussion......Page 596
Acknowledgments......Page 598
Literature Cited......Page 599
29 Mechanisms and Rate Laws in Electrolyte Crystal Growth from Aqueous Solution......Page 600
Transport Controlled Kinetics......Page 601
Surface Controlled Kinetics......Page 604
Estimates of Parameters......Page 606
Prediction of Growth Rates......Page 607
Discussion......Page 610
Conclusion......Page 611
Legend of Symbols......Page 612
Literature Cited......Page 613
30 Influence of Surface Area, Surface Characteristics, and Solution Composition on Feldspar Weathering Rates......Page 615
Parabolic Kinetics......Page 616
Petrographic Observations......Page 620
Surface Composition......Page 623
Laboratory Studies of Feldspar Weathering - A Summary......Page 625
Rates of Feldspar Weathering in the Laboratory......Page 626
Rates of Feldspar Weathering in Natural Weathering Profiles from Geochemical Mass Balance of Small Watersheds......Page 627
Summary and Evaluation......Page 629
Conclusions......Page 631
Literature Cited......Page 632
31 Dislocation Etch Pits in Quartz......Page 635
Theory of Etch Pit Formation......Page 636
Etch Pits in Quartz: A Test of the Theory......Page 639
Conclusions and Implications for Future Research......Page 645
Legend of Symbols......Page 647
Literature Cited......Page 648
32 The Growth of Calcium Phosphates......Page 650
Experimental......Page 653
Results and Discussions......Page 654
Literature Cited......Page 661
Author Index......Page 663
A......Page 664
C......Page 666
D......Page 668
E......Page 669
G......Page 671
I......Page 672
K......Page 673
M......Page 674
O......Page 676
P......Page 677
R......Page 679
S......Page 680
T......Page 683
Z......Page 684

Citation preview

Geochemical Processes at Mineral Surfaces

In Geochemical Processes at Mineral Surfaces; Davis, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

In Geochemical Processes at Mineral Surfaces; Davis, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

ACS

SYMPOSIUM

SERIES

Geochemical Processes at Mineral Surfaces James A . Davis, EDITOR U.S. Geological Survey Ki Stanford University

Developed from a symposium sponsored by the Division of Environmental Chemistry and the Division of Geochemistry at the 190th Meeting of the American Chemical Society, Chicago, Illinois, September 8-13, 1985

American Chemical Society, Washington, D C 1986

In Geochemical Processes at Mineral Surfaces; Davis, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

323

Library of Congress Cataloging-in-Publication Data Geochemical processes at mineral surfaces. (ACS symposium series, ISSN 0097-6156; 323) "Developed from a symposium sponsored by the Division of Environmental Chemistry and the Division of Geochemistry at the 190th Meeting of the American Chemical Society, Chicago, Illinois, September 8-13, 1985." Bibliography: p. Includes indexes. 1. Geochemistry—Congresses. 2. Mineralogical chemistry—Congresses. I. Davis, James Α., 1950. 1953. III. American Chemica Society Environmental Chemistry. IV. American Chemical Society. Division of Geochemistry. V. American Chemical Society. Meeting (190th: 1985: Chicago, 111.) VI. Series. QE515.G3724 1986 551.9 86-22173 ISBN 0-8412-1004-7

Copyright © 1986 American Chemical Society All Rights Reserved. The appearance of the code at the bottom of the first page of each chapter in this volume indicates the copyright owner's consent that reprographic copies of the chapter may be made for personal or internal use or for the personal or internal use of specific clients. This consent is given on the condition, however, that the copier pay the stated per copy fee through the Copyright Clearance Center, Inc., 27 Congress Street, Salem, MA 01970, for copying beyond that permitted by Sections 107 or 108 of the U.S. Copyright Law. This consent does not extend to copying or transmission by any means—graphic or electronic—for any other purpose, such as for general distribution, for advertising or promotional purposes, for creating a new collective work, for resale, or for information storage and retrieval systems. The copying fee for each chapter is indicated in the code at the bottom of the first page of the chapter. The citation of trade names and/or names of manufacturers in this publication is not to be construed as an endorsement or as approval by ACS of the commercial products or services referenced herein; nor should the mere reference herein to any drawing, specification, chemical process, or other data be regarded as a license or as a conveyance of any right or permission, to the holder, reader, or any other person or corporation, to manufacture, reproduce, use, or sell any patented invention or copyrighted work that may in any way be related thereto. Registered names, trademarks, etc., used in this publication, even without specific indication thereof, are not to be considered unprotected by law. PRINTED IN THE UNITED STATES OF AMERICA

In Geochemical Processes at Mineral Surfaces; Davis, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

ACS Symposium Series M. Joan Comstock, Series Editor

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In Geochemical Processes at Mineral Surfaces; Davis, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

FOREWORD The ACS SYMPOSIUM m e d i u m for p u b l i s h i n g s y m p o s i a q u i c k l y i n b o o k f o r m . T h e format o f the Series parallels that of the c o n t i n u i n g ADVANCES IN CHEMISTRY SERIES except that, i n order to save time, the papers are not typeset but are reproduced as they are submitted by the authors i n camera-ready f o r m . Papers are reviewed under the supervision of the E d i t o r s w i t h the assistance o f the Series A d v i s o r y B o a r d a n d are selected to m a i n t a i n the integrity of the s y m p o s i a ; however, v e r b a t i m reproductions o f previously p u b lished papers are not accepted. B o t h reviews a n d reports o f research are acceptable, because s y m p o s i a m a y embrace b o t h types of presentation.

In Geochemical Processes at Mineral Surfaces; Davis, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

PREFACE THIS B O O K deals only with the chemistry of the mineral-water interface, and so at first glance, the book might appear to have a relatively narrow focus. However, the range of chemical and physical processes considered is actually quite broad, and the general and comprehensive nature of the topics makes this volume unique. The technical papers are organized into physical properties of the mineral-water interface; adsorption; ion exchange; surface spectroscopy; dissolution, precipitation, and solid solution formation; and transformation reactions at the mineral-water interface. The introductory chapter presents an overview of recent research advances in each of these six areas an paper. Several papers address the complex ways in which some processes are interrelated, for example, the effect of adsorption reactions on the catalysis of electron transfer reactions by mineral surfaces. Papers in the symposium upon which this book is based were contributed by a diverse group of scientists from the fields of geochemistry, hydrogeology, water chemistry, chemical engineering, soil chemistry, electrical engineering, dental science, and mathematics. This diversity was facilitated by joint sponsorship of the symposium by the Environmental Chemistry and Geochemistry Divisions of A C S . The participants shared a common interest in the chemistry of mineral-water interfaces; however, each of these scientific disciplines has a different perspective on the theories and experimental methods that are used to describe interfacial chemistry. Research publications on the topic are widely dispersed in the literature, and as a result, many investigators are unaware of recent advances in disciplines other than their own. Thus, it appeared important and timely to compile recent developments in these related fields in a single volume. It is hoped that this effort will contribute to a greater understanding of the significant role of interfacial reactions in geochemical processes. The inspiration for this symposium emerged from stimulating discussions with colleagues at the Gordon Conference on Environmental Sciences: Water held at New Hampton, N.H., in June 1984 (chaired by C . R. O'Melia). Several people assisted with the selection of speakers, including J. O. Leckie, L. N. Plummer, G . A. Parks, D. D. Eberl, A. F. White, A. T. Stone, and J. C . Westall. Financial support for foreign speakers was provided by the A C S Petroleum Research Fund and each of the sponsoring A C S divisions. The quality of this volume is due in part to the careful work of

xi

In Geochemical Processes at Mineral Surfaces; Davis, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

numerous technical reviewers who submitted detailed comments and criticisms, and we are greatly indebted to these reviewers. We also thank Robin Giroux of the ACS Books Department for her guidance throughout the editorial process. JAMES A. DAVIS

U.S. Geological Survey Menlo Park, CA 94025 KIM F. HAYES

Stanford University Stanford, CA 94305-4020 June 1, 1986

In Geochemical Processes at Mineral Surfaces; Davis, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

1

Geochemical Processes at M i n e r a l Surfaces: An Overview 1

2

James A. Davis and Kim F. Hayes 1

Water Resources Division, U.S. Geological Survey, Menlo Park, CA 94025 Environmental Engineering and Science Group, Department of Civil Engineering, Stanford University, Stanford, CA 94305-4020

2

The phase discontinuit greatly influences th geochemica cycle y composition of natural waters and the flux of material through the hydrosphere are largely controlled by the weathering of minerals and the precipitation of new phases - - processes in which the mineral water interface plays a fundamental role. In addition, mineral surfaces may act as catalysts for chemical or biological transformations that occur within the hydrosphere. Reactions at the mineral-water interface are of interest in the study of ore genesis, geochemical exploration, mineral separation processes such as flotation and sedimentation, transport of adsorbed nutrients or pollutants in rivers and lakes, scavenging of trace elements in the oceans, and the transport of nuclear or other hazardous waste materials in groundwaters. Because these processes are so complex, we rely to a great extent on models to understand our observations of the geochemical behavior of solutes in water. The models typically contain several components; for example, solute transport models may be composed of a hydrologie model coupled with a chemical or biological submodel. The chemical submodel can be as simple as a distribution coefficient to represent the partitioning of an element between solid and aqueous phases, or i f the aqueous chemical composition is expected to be controlled by the dissolution of a mineral, a solubility product or rate constant for dissolution would be included in the model. Ideally, the chemical model would describe geochemical behavior in terms of a series or combination of elementary processes, e.g. adsorption, ion exchange, precipitation, dissolution, or electron transfer reactions. In reality, the behavior of geochemical systems is often so complex that the actual mechanisms of the processes observed are not well understood. In this overview we discuss recent advances in the study of chemical reactions at the mineral-water interface as we introduce the 0097-6156/ 86/ 0323-0002S06.00/ 0 © 1986 American Chemical Society

In Geochemical Processes at Mineral Surfaces; Davis, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

1.

DAVIS A N D HAYES

Overview

3

c h a p t e r s c o n t a i n e d i n t h i s v o l u m e . Our o b j e c t i v e i s t o i n d i c a t e i m p o r t a n t f e a t u r e s of each c h a p t e r to the f i e l d of aqueous g e o c h e m i s t r y and ways i n which the c h a p t e r s r e l a t e t o each o t h e r . The p a p e r i s d i v i d e d i n t o s e c t i o n s on p h y s i c a l p r o p e r t i e s o f t h e interface; a d s o r p t i o n and i o n e x c h a n g e ; surface spectroscopy; d i s s o l u t i o n , p r e c i p i t a t i o n , and s o l i d s o l u t i o n f o r m a t i o n ; and t r a n s f o r m a t i o n r e a c t i o n s a t the m i n e r a l - w a t e r i n t e r f a c e . Each s e c t i o n touches on the importance o f t h a t t o p i c i n geochemical p r o c e s s e s and i n t e r d e p e n d e n t r e l a t i o n s h i p s among the t o p i c s c o v e r e d . Physical Properties of the Mineral-Water Interface The c h e m i c a l r e a c t i v i t y o f t h e m i n e r a l - w a t e r i n t e r f a c e i s i n f l u e n c e d by some i m p o r t a n t p r o p e r t i e s w h i c h d i s t i n g u i s h t h e i n t e r f a c i a l e n v i r o n m e n t from t h a t o f b u l k w a t e r , e.g. 1) water i s more s t r u c t u r e d i n t h e i n t e r f a c i a l r e g i o n , 2) i o n s and w a t e r m o l e c u l e s a r e l e s s m o b i l e , 3) t h e d i e l e c t r i c c o n s t a n t o f w a t e r i s d e c r e a s e d , and 4) e l e c t r i c a l charge and p o t e n t i a t o t h e f o r m a t i o n o f an e l e c t r i c a s t u d i e s o f m i n e r a l - w a t e r i n t e r f a c e have been e x t e n s i v e , the e f f e c t s o f t h e p e r t u r b e d l a y e r o f water and t h e e l e c t r i c a l d o u b l e l a y e r on c h e m i c a l r e a c t i o n s at the i n t e r f a c e are s t i l l u n r e s o l v e d i s s u e s (l11). M u l l a ( C h a p t e r 2) c o m p a r e s s i m u l a t i o n s o f i n t e r f a c i a l w a t e r s t r u c t u r e from v a r i o u s s t a t i s t i c a l - m e c h a n i c a l m o d e l s , i n c l u d i n g Monte C a r l o and M o l e c u l a r D y n a m i c s m o d e l s . The r e s u l t s p r e d i c t t h e e x i s t e n c e o f m o l e c u l a r l a y e r i n g w i t h o r d e r e d d i p o l e o r i e n t a t i o n s at the i n t e r f a c e , d e n s i t y o s c i l l a t i o n s which extend many Angstroms away from t h e s u r f a c e , and fewer hydrogen bonds between w a t e r m o l e c u l e s i n the i n t e r f a c i a l r e g i o n . The e x i s t e n c e o f d e n s i t y o s c i l l a t i o n s at the i n t e r f a c e has r e c e n t l y been c o n f i r m e d e x p e r i m e n t a l l y (12). Reduced d i p o l e r e l a x a t i o n times are a l s o p r e d i c t e d , which suggests t h a t i n t e r f a c i a l w a t e r e x p e r i e n c e s h i n d e r e d r o t a t i o n . U n f o r t u n a t e l y the d i e l e c t r i c p r o p e r t i e s cannot be e f f e c t i v e l y modeled, but the r e s u l t s do s u g g e s t t h a t i n t e r f a c i a l w a t e r i s n o t a u n i f o r m d i e l e c t r i c continuum. The development o f improved models i n t h e f u t u r e appears p r o m i s i n g , and t h e s e m o d e l s s h o u l d i n c r e a s e o u r u n d e r s t a n d i n g o f p r o p e r t i e s w h i c h are not e a s i l y q u a n t i f i e d at p r e s e n t , e.g. h y d r a t i o n f o r c e s , hydrophobic e f f e c t s , and d o u b l e l a y e r f o r c e s . G i e s e and C o n s t a n z o ( C h a p t e r 3) p r e s e n t t h e r e s u l t s o f an infrared study of the bonding of i n t e r c a l a t e d water i n s y n t h e t i c h y d r a t e d k a o l i n i t e s . Two t y p e s o f w a t e r were i d e n t i f i e d : 1) w a t e r m o l e c u l e s which were bonded t o the d i t r i g o n a l h o l e s o f the s i l i c a t e l a y e r , and 2) a s s o c i a t e d water m o l e c u l e s which were hydrogen bonded t o t h o s e o f the f i r s t group. The m o b i l i t y o f water m o l e c u l e s bound t o t h e d i t r i g o n a l h o l e s was g r e a t l y r e d u c e d i n c o m p a r i s o n t o t h e a s s o c i a t e d water. A l t h o u g h h y d r a t e d k a o l i n i t e s are not found i n n a t u r e , G i e s e and C o n s t a n z o s u g g e s t t h a t t h e surface-water i n t e r a c t i o n s observed in t h i s study are r e p r e s e n t a t i v e of i n t e r a c t i o n s w i t h the s u r f a c e s o f o t h e r s i l i c a t e m i n e r a l s . The p r o p e r t i e s o f t h e e l e c t r i c a l d o u b l e l a y e r (EDL) h a v e been the s u b j e c t o f c o n s i d e r a b l e r e s e a r c h (1,3,5,8,10). U n l i k e r e v e r s i b l e e l e c t r o d e s , where s u r f a c e p o t e n t i a l i s c o n t r o l l e d and charge d e v e l o p s i n response to changes i n e l e c t r o d e p o t e n t i a l , m i n e r a l s u r f a c e s d e v e l o p p o t e n t i a l i n response t o the f o r m a t i o n o f s u r f a c e charge (8). On t h e s u r f a c e o f h y d r o u s o x i d e s , f o r e x a m p l e , h y d r o x y l g r o u p s

In Geochemical Processes at Mineral Surfaces; Davis, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

4

G E O C H E M I C A L P R O C E S S E S AT M I N E R A L S U R F A C E S

( B r o n s t e d a c i d s i t e s ) o r m e t a l atoms w i t h u n s a t i s f i e d c o o r d i n a t i o n ( L e w i s a c i d s i t e s ) r e a c t w i t h w a t e r t o f o r m s u r f a c e c h a r g e (1.3). Isomorphic s u b s t i t u t i o n i n the i n t e r l a y e r r e g i o n o f l a y e r e d s i l i c a t e s r e s u l t s in a n e g a t i v e surface charge. In e a c h c a s e c h e m i c a l "exchange" o f i o n s between phases r e s u l t s i n the f o r m a t i o n o f s u r f a c e charge and the development o f an e l e c t r i c a l p o t e n t i a l . I t i s i m p o r t a n t t o e s t a b l i s h t h e o r i g i n and m a g n i t u d e o f t h e a c i d i t y (and h e n c e , t h e c h a r g e ) o f m i n e r a l s u r f a c e s , b e c a u s e t h e r e a c t i v i t y of the surface i s d i r e c t l y r e l a t e d to i t s a c i d i t y . Several m i c r o s c o p i c - m e c h a n i s t i c m o d e l s h a v e been p r o p o s e d t o d e s c r i b e t h e a c i d i t y o f h y d r o x y l g r o u p s on o x i d e s u r f a c e s ; most d e s c r i b e t h e s u r f a c e i n terms o f amphoteric weak a c i d groups (14-17), but r e c e n t l y a m o n o p r o t i c weak a c i d model f o r t h e s u r f a c e was p r o p o s e d ( ! 8 ) . The m o d e l s d i f f e r p r i m a r i l y i n t h e i r d e s c r i p t i o n o f t h e EDL and t h e a s s u m p t i o n s used to d e s c r i b e i n t e r f a c i a l s t r u c t u r e . " I n t r i n s i c " a c i d i t y c o n s t a n t s t h a t are d e r i v e d from t h e s e models can have s u b s t a n t i a l l y d i f f e r e n t v a l u e s because o f the d i f f e r e n t assumptions e m p l o y e d i n e a c h model f o r t h e s t r u c t u r e o f t h e EDL (5). W e s t a l l ( C h a p t e r 4) r e v i e w s s e v e r a d e s c r i b e the a c i d i t y o f o x i d e s u r f a c e s and compares t h e a p p l i c a b i l i t y o f t h e s e models w i t h the m o n o p r o t i c weak a c i d m o d e l . The assumptions employed by each o f the models t o e s t i m a t e v a l u e s o f thermodynamic c o n s t a n t s are c r i t i c a l l y examined. The d i f f i c u l t y i n c h a r a c t e r i z i n g the i n t e r f a c e a r i s e s from the f a c t t h a t t h e e l e c t r o s t a t i c i n t e r a c t i o n s are c l o s e l y c o u p l e d t o the c h e m i c a l i n t e r a c t i o n s . An i n d e p e n d e n t measurement o f e l e c t r o s t a t i c energy w o u l d be u s e f u l f o r p r o b i n g the s e p a r a t i o n o f c o u l o m b i c and c h e m i c a l components i n the EDL models. Bousse and M e i n d l (Chapter 5) d e s c r i b e a t e c h n i q u e f o r measuring the e l e c t r i c a l p o t e n t i a l a t o x i d e s u r f a c e s u s i n g i o n - s e n s i t i v e f i e l d e f f e c t t r a n s i t o r s ( I S F E T s ) . In t h i s method one may r e g u l a t e t o t a l e l e c t r i c a l p o t e n t i a l o f t h e i n t e r f a c e , and t h i s a l l o w s e s t i m a t e s o f i n t r i n s i c a c i d i t y c o n s t a n t s t h a t a r e i n d e p e n d e n t o f p r o t o n a d s o r p t i o n d a t a . The m e a s u r e m e n t o f the t o t a l e l e c t r i c a l p o t e n t i a l i s p r e f e r a b l e to t h a t of zeta p o t e n t i a l , s i n c e t h e e x a c t l o c a t i o n o f t h e l a t t e r measurement i s i n d e t e r m i n a t e . F u r t h e r m o r e , i t has been shown t h a t t h e dependence o f p r o t o n a d s o r p t i o n as a f u n c t i o n o f pH may depend t o a g r e a t degree on the extent of complex formation w i t h adsorbed c o u n t e r i o n s (19-21), w h e r e a s t h e t o t a l p o t e n t i a l as a f u n c t i o n o f pH i s r e l a t i v e l y i n s e n s i t i v e t o c o m p l e x a t i o n (21). Chan ( C h a p t e r 6) p r e s e n t s a s i m p l e g r a p h i c a l method f o r e s t i m a t i n g t h e f r e e e n e r g y o f EDL f o r m a t i o n a t t h e o x i d e - w a t e r i n t e r f a c e w i t h an amphoteric model f o r the a c i d i t y o f s u r f a c e groups. S u b j e c t t o t h e a s s u m p t i o n s o f t h e EDL m o d e l , t h e g r a p h i c a l method a l l o w s a comparison o f the magnitudes o f the c h e m i c a l and c o u l o m b i c components o f s u r f a c e r e a c t i o n s . The a n a l y s i s a l s o i l l u s t r a t e s the r e l a t i o n s h i p b e t w e e n model p a r a m e t e r v a l u e s and t h e d e v i a t i o n o f s u r f a c e p o t e n t i a l from the N e r n s t e q u a t i o n . The r e l a t i v e i m p o r t a n c e o f t h e EDL f o r r e a c t i o n s o t h e r t h a n a d s o r p t i o n i s not w e l l u n d e r s t o o d . S u r f a c e c o m p l e x a t i o n models have r e c e n t l y been a p p l i e d t o p r o c e s s e s i n which a d s o r p t i o n r e p r e s e n t s the f i r s t s t e p i n a sequence o f r e a c t i o n s . For example, Stumm e t a l . (22) h a v e a p p l i e d a model w i t h an EDL component i n t h e i r s t u d i e s o f t h e r o l e o f a d s o r p t i o n i n d i s s o l u t i o n and p r e c i p i t a t i o n r e a c t i o n s . The e f f e c t o f s u r f a c e c h a r g e and p o t e n t i a l on p r e c i p i t a t i o n and t h e

In Geochemical Processes at Mineral Surfaces; Davis, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

1.

DAVIS A N D HAYES

Overview

5

f o r m a t i o n o f m e t a s t a b l e s o l i d phases i s d i s c u s s e d by Zawacki et a l . ( C h a p t e r 3 2 ) . W a i t e ( C h a p t e r 20) r e v i e w s t h e i m p o r t a n t r o l e o f t h e i n t e r f a c i a l environment i n c a t a l y z i n g l i g h t - i n d u c e d redox r e a c t i o n s . V o u d r i a s and R e i n h a r d ( C h a p t e r 22) d i s c u s s t h e e f f e c t s o f s u r f a c e a c i d i t y on t r a n s f o r m a t i o n s o f o r g a n i c compounds. The r a t e s o f c h e m i c a l r e a c t i o n s a r e a l s o i n f l u e n c e d by t h e E D L . A s w i t h e q u i l i b r i u m models, the r e l a t i v e c o n t r i b u t i o n o f t h e EDL i n d e t e r m i n i n g the overall r e a c t i o n r a t e i s d e p e n d e n t on t h e i n t e r f a c i a l model chosen (see Chapters 7 and 12). Adsorption S o r p t i o n p r o c e s s e s h a v e r e c e i v e d much s t u d y b e c a u s e o f t h e i r fundamental importance i n g e o c h e m i s t r y , a n a l y t i c a l c h e m i s t r y , and i n i n d u s t r i a l a p p l i c a t i o n s . The t h r e e p r i n c i p a l s o r p t i o n p r o c e s s e s are a d s o r p t i o n , a b s o r p t i o n , and s u r f a c e p r e c i p i t a t i o n ; t h e d i f f e r e n c e s among t h e s e p r o c e s s e s a r e d i s c u s s e d by S p o s i t o ( C h a p t e r 1 1 ) . I f t h e s p e c i f i c process leadin s o l u t i o n i s not known, the The abundance o f l i t e r a t u r e on e x p e r i m e n t a l s t u d i e s o f i o n a d s o r p t i o n has been r e v i e w e d by K i n n i b u r g h and J a c k s o n (23) and H i n g s t o n ( 2 4 ) . R e c e n t l a b o r a t o r y s t u d i e s o f s o r p t i o n p r o c e s s e s have been d i r e c t e d more toward a b e t t e r u n d e r s t a n d i n g o f the mechanisms o f r e a c t i o n s , a c h a r a c t e r i z a t i o n o f the bonding o f adsorbed s p e c i e s , and improvements in a d s o r p t i o n models. It i s i n c r e a s i n g l y c l e a r , however, t h a t the m a c r o s c o p i c approaches t h a t have been commonly used t o s t u d y s o r p t i o n p r o c e s s e s , e.g. a d s o r p t i o n i s o t h e r m s , s o l u b i 1 i t y c a l c u l a t i o n s , and k i n e t i c methods, cannot u n e q u i v o c a b l y d i s t i n g u i s h between p r o c e s s e s such as a d s o r p t i o n and s u r f a c e p r e c i p i t a t i o n (see C h a p t e r 11). I t i s l i k e l y t h a t f u t u r e developments i n t h i s f i e l d w i l l come from s t u d i e s u t i l i z i n g m o l e c u l a r t e c h n i q u e s such as i n - s i t u s u r f a c e s p e c t r o s c o p y . Mechanisms of Sorption Processes. K i n e t i c s t u d i e s are v a l u a b l e f o r h y p o t h e s i z i n g m e c h a n i s m s o f r e a c t i o n s i n homogeneous s o l u t i o n , but the i n t e r p r e t a t i o n o f k i n e t i c d a t a f o r s o r p t i o n p r o c e s s e s i s more d i f f i c u l t . R e c e n t l y i t has been shown t h a t t h e m e c h a n i s m s o f v e r y f a s t a d s o r p t i o n r e a c t i o n s may be i n t e r p r e t e d f r o m t h e r e s u l t s o f c h e m i c a l r e l a x a t i o n s t u d i e s (25-27). Yasunaga and Ikeda (Chapter 12) summarize r e c e n t s t u d i e s t h a t have u t i l i z e d r e l a x a t i o n t e c h n i q u e s t o e x a m i n e t h e a d s o r p t i o n o f c a t i o n s and a n i o n s on h y d r o u s o x i d e and a l u m i n o s i l i c a t e s u r f a c e s . Hayes and L e c k i e ( C h a p t e r 7) p r e s e n t new i n t e r p r e t a t i o n s f o r the mechanism o f l e a d i o n a d s o r p t i o n by g o e t h i t e . In b o t h p a p e r s i t i s c o n c l u d e d t h a t t h e k i n e t i c and e q u i l i b r i u m a d s o r p t i o n d a t a a r e c o n s i s t e n t w i t h the r a t e r e l a t i o n s h i p s d e r i v e d from an i n t e r f a c i a l model i n which metal i o n s are l o c a t e d n e a r e r t o the s u r f a c e than adsorbed c o u n t e r i o n s . The s u r f a c e s o f m i n e r a l s are g e n e r a l l y not homogeneous; k i n k s , s t e p s , e d g e s , d i s l o c a t i o n s , o r p o i n t d e f e c t s may p r o v i d e r e a c t i v e zones. M i c r o c r y s t a l 1 i n e p r e p a r a t i o n s o f s o l i d s , which are f r e q u e n t l y used i n l a b o r a t o r y s o r p t i o n e x p e r i m e n t s , may have s e v e r a l c l e a v a g e p l a n e s w i t h d i f f e r e n t s i t e e n e r g i e s . The i m p o r t a n c e o f t h e h i g h energy s i t e s t h a t r e s u l t from t h e s e i m p e r f e c t i o n s i s w e l l r e c o g n i z e d f o r t h e p r o c e s s e s o f d i s s o l u t i o n and c r y s t a l g r o w t h (28). S e v e r a l e q u i l i b r i u m s t u d i e s of i o n a d s o r p t i o n have suggested t h a t hydrous o x i d e s u r f a c e s a r e composed o f h e t e r o g e n e o u s s i t e s ( 2 9 - 3 2 ) . Some p r e l i m i n a r y r e s u l t s from a k i n e t i c study t h a t suggest the e x i s t e n c e

In Geochemical Processes at Mineral Surfaces; Davis, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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G E O C H E M I C A L P R O C E S S E S AT M I N E R A L S U R F A C E S

o f heterogeneous s i t e s on the s u r f a c e o f g o e t h i t e are g i v e n i n Chapter 7. D e s p i t e the growing e v i d e n c e o f s u r f a c e s i t e h e t e r o g e n e i t y , most s u r f a c e c o m p l e x a t i o n models are based on the concept o f a homogeneous s u r f a c e w i t h a v e r a g e d EDL p r o p e r t i e s ( 5 , 8 , 1 8 , 3 3 - 3 8 ) . In a d d i t i o n to h e t e r o g e n e o u s s i t e s , m i n e r a l s u r f a c e s may c o n t a i n e i t h e r p o o r l y c r y s t a l 1 i z e d or wel 1 h y d r a t e d m a t e r i a l . Thus, m u l t i p l e s o r p t i o n mechanisms may o p e r a t e at the b e g i n n i n g o f many l a b o r a t o r y s t u d i e s o f s o r p t i o n k i n e t i c s . S p o s i t o (Chapter 11) argues t h a t t h i s m u l t i p l i c i t y prevents a simple i n t e r p r e t a t i o n of k i n e t i c or e q u i l i b r i u m data in terms o f a s i m p l e f i r s t o r d e r r a t e l a w o r an a d s o r p t i o n mechanism. R e l a x a t i o n s t u d i e s have shown t h a t the attachment o f an i o n t o a s u r f a c e i s v e r y f a s t , but the e s t a b l i s h m e n t o f e q u i l i b r i u m i n w e l l d i s p e r s e d s u s p e n s i o n s o f c o l l o i d a l p a r t i c l e s i s much s l o w e r . A d s o r p t i o n o f c a t i o n s by h y d r o u s o x i d e s may a p p r o a c h e q u i l i b r i u m w i t h i n a m a t t e r o f m i n u t e s i n some s y s t e m s ( 3 9 - 4 0 ) . H o w e v e r , c a t i o n and a n i o n s o r p t i o n p r o c e s s e s o f t e n e x h i b i t a r a p i d i n i t i a l stage of a d s o r p t i o n t h a t i s f o l l o w e d by a much s l o w e r r a t e o f uptake (24,4143). S e v e r a l s t u d i e s o i o n s between aqueous s o l u t i o n t h a t the k i n e t i c s of phosphate d e s o r p t i o n are very slow (43-45). Numerous hypotheses have been suggested f o r t h i s slow attainment of e q u i l i b r i u m i n c l u d i n g 1) the f o r m a t i o n o f b i n u c l e a r complexes on the s u r f a c e (44); 2) dynamic p a r t i c l e - p a r t i c l e i n t e r a c t i o n s i n which an a d s o r b i n g i o n enhances c o n t a c t adhesion between p a r t i c l e s (43,45-46); 3) d i f f u s i o n o f i o n s i n t o a d s o r b e n t s ( 4 7 ) ; a n d 4) surface p r e c i p i t a t i o n (48-50). Bleam and M c B r i d e (51-52) r e c e n t l y p r e s e n t e d e v i d e n c e t h a t the a r r a n g e m e n t o f g r o u p s o f s i t e s on m i n e r a l s u r f a c e s may i n f l u e n c e a d s o r p t i o n . These a u t h o r s argued t h a t , under c e r t a i n c o n d i t i o n s , the f o r m a t i o n o f a monolayer o f a d s o r b i n g i o n s may be l e s s f a v o r a b l e than the f o r m a t i o n o f a m u l t i l a y e r c l u s t e r o f p o l y m e r i z e d o r p r e c i p i t a t e d m a t e r i a l . S e v e r a l s t u d i e s h a v e i n d i c a t e d t h a t a d s o r p t i o n may be d e s c r i b e d by c o m p l e x a t i o n r e a c t i o n s a t d i s c r e t e s u r f a c e s i t e s a t low s u r f a c e c o v e r a g e , but t h a t p o l y m e r i z a t i o n and h y d r o x i d e p r e c i p i t a t i o n may o c c u r a t h i g h s u r f a c e c o v e r a g e ( 5 3 - 5 5 ) . F a r l e y e t a l . (56) r e c e n t l y proposed a model f o r s o r p t i o n o f c a t i o n s on hydrous o x i d e s t h a t a l l o w s f o r a c o n t i n u u m b e t w e e n a d s o r p t i o n and s u r f a c e precipitation as t h e s o r p t i o n d e n s i t y i n c r e a s e s . Surface c o p r e c i p i t a t e s ( s o l i d s o l u t i o n s ) may form when an a d s o r b i n g c a t i o n i s capable of occupying s t r u c t u r a l s i t e s i n the adsorbent l a t t i c e . Experimental evidence o f t h i s t y p e o f p r o c e s s has been g i v e n by M c B r i d e (57) f o r a l u m i n a and by D a v i s e t a l . (49) f o r c a l c i t e . S p e c t r o s c o p i c t e c h n i q u e s may p r o v i d e t h e l e a s t ambiguous methods f o r v e r i f i c a t i o n of a c t u a l s o r p t i o n mechanisms. Z e l t n e r et a l . ( C h a p t e r 8) h a v e a p p l i e d F T I R ( F o u r i e r T r a n s f o r m Infrared) s p e c t r o s c o p y and m i c r o c a l o r i m e t r i c t i t r a t i o n s i n a s t u d y o f t h e a d s o r p t i o n o f s a l i c y l i c a c i d by g o e t h i t e ; t h e s e t e c h n i q u e s p r o v i d e new i n f o r m a t i o n on the s t r u c t u r e o f o r g a n i c a c i d complexes formed at t h e g o e t h i t e - w a t e r i n t e r f a c e . Ambe e t a l . ( C h a p t e r 19) p r e s e n t t h e r e s u l t s o f an e m i s s i o n Mossbauer s p e c t r o s c o p i c s t u d y o f sorbed Co(II) and Sb(V). A l t h o u g h Mossbauer s p e c t r o s c o p y can o n l y be used f o r a few c h e m i c a l e l e m e n t s , the t e c h n i q u e p r o v i d e s d e t a i l e d i n f o r m a t i o n about t h e m o l e c u l a r b o n d i n g o f s o r b e d s p e c i e s a n d may be u s e d t o d i f f e r e n t i a t e between a d s o r p t i o n and s u r f a c e p r e c i p i t a t i o n .

In Geochemical Processes at Mineral Surfaces; Davis, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

1.

DAVIS A N D HAYES

Overview

1

E m p i r i c a l models. N a t u r a l s y s t e m s c o n t a i n a w i d e v a r i e t y o f mineral s u r f a c e s t h a t may be i n v o l v e d i n s o r p t i o n processes. A p p l i c a t i o n of surface complexation models r e q u i r e s a d e t a i l e d c h a r a c t e r i z a t i o n of each a d s o r b e n t p r e s e n t , and t h e amount o f i n f o r m a t i o n r e q u i r e d g e n e r a l l y exceeds our knowledge of the p r o p e r t i e s o f n a t u r a l m a t e r i a l s (58). W h i l e t h e m o d e l s h a v e been m o d e r a t e l y s u c c e s s f u l i n d e s c r i b i n g the r e s u l t s from l a b o r a t o r y s t u d i e s w i t h p u r e m i n e r a l p h a s e s (e.g. 5 9 ) , t h e y h a v e n o t y e t been a p p l i e d i n f i e l d s t u d i e s . Some a u t h o r s (58,60) have made c a l c u l a t i o n s for hypothetical s e d i m e n t s f o r p r e d i c t i v e p u r p o s e s . In t h e s e c a l c u l a t i o n s the c o n v e n t i o n a l approach has been t o assume t h a t the overall a d s o r p t i o n o f an i o n f o r a m i x t u r e o f m i n e r a l s c a n be d e s c r i b e d as the sum o f the a d s o r p t i v e c o n t r i b u t i o n o f each m i n e r a l . However, Honeyman (61.) has d e m o n s t r a t e d t h a t t h i s c o n c e p t of "adsorptive a d d i t i v i t y " does not h o l d , even i n s i m p l e experiments w i t h b i n a r y m i x t u r e s o f o x i d e p h a s e s . In t h e a b s e n c e o f a u n i f i e d t h e o r e t i c a l m o d e l , g e o c h e m i s t s have o f t e n f o r m u l a t e d e m p i r i c a l approaches t h a t u t i l i z a d s o r p t i o n process (60,62-63) t h a t t h e s e m a c r o s c o p i c p a r a m e t e r s a r e t h e n e t r e s u l t o f numerous m i c r o s c o p i c s u b r e a c t i o n s o c c u r r i n g i n t h e s y s t e m . In p a r t i c u l a r , Honeyman and L e c k i e r e v i e w the use o f m a c r o s c o p i c p r o t o n c o e f f i c i e n t s (e.g. Kurbatov c o e f f i c i e n t s ) i n p r a c t i c a l s o r p t i o n m o d e l s . The a u t h o r s show t h a t m a c r o s c o p i c p r o t o n c o e f f i c i e n t s are r a r e l y o b s e r v e d t o have i n t e g r a l v a l u e s , d e s p i t e the f a c t t h a t p r o t o n c o e f f i c i e n t s o f t h e m i c r o s c o p i c a d s o r p t i o n r e a c t i o n s o f i n t e r e s t may have i n t e g r a l v a l u e s . A mathematical d e r i v a t i o n supporting t h i s c o n c l u s i o n is p r e s e n t e d i n Chapter 7. S o r p t i o n of o r g a n i c compounds. A d s o r p t i o n may p l a y an i m p o r t a n t r o l e i n the t r a n s f o r m a t i o n s o f o r g a n i c compounds i n t h e e n v i r o n m e n t . For example, a d s o r p t i o n o f o r g a n i c p o l l u t a n t s a t t h e m i n e r a l - w a t e r i n t e r f a c e may c a t a l y z e t h e c o n v e r s i o n o f t h e s e compounds t o l e s s harmful p r o d u c t s . The c h e m i c a l f a c t o r s c o n t r o l l i n g t h e s o r p t i o n o f h y d r o p h o b i c compounds i n s e d i m e n t s w i t h m o d e r a t e t o h i g h o r g a n i c c o n t e n t have r e c e i v e d c o n s i d e r a b l e s t u d y (64), but s o r p t i o n p r o c e s s e s f o r sediments c o n t a i n i n g low o r g a n i c c o n t e n t are p o o r l y u n d e r s t o o d . C u r t i s e t a l . (Chapter 10) d i s c u s s c u r r e n t problems i n u n d e r s t a n d i n g the s o r p t i o n b e h a v i o r o f h y d r o p h o b i c o r g a n i c compounds, i n c l u d i n g r e a c t i o n k i n e t i c s , h y s t e r e s i s e f f e c t s , and the i n f l u e n c e o f d i s s o l v e d macromolecular organic m a t e r i a l . Ion Exchange The d i s t r i b u t i o n o f major e l e m e n t s between s o i l s and s o i l s o l u t i o n s i s known t o be g o v e r n e d p r i m a r i l y by i o n e x c h a n g e p r o c e s s e s ( 6 5 ) . These p r o c e s s e s are i m p o r t a n t because t h e y g r e a t l y i n f l u e n c e the uptake o f n u t r i e n t s by p l a n t s and o t h e r l i v i n g o r g a n i s m s . Even though an i o n exchange r e a c t i o n c o u l d be c l a s s i f i e d as a t y p e o f a d s o r p t i o n r e a c t i o n , i t i s u s u a l l y t r e a t e d as a s e p a r a t e s o r p t i o n p r o c e s s as a m a t t e r o f c o n v e n i e n c e and t r a d i t i o n . D e s c r i b i n g i o n exchange r e a c t i o n s as t h o s e t a k i n g p l a c e a t " c o n s t a n t c h a r g e " s u r f a c e s , e.g. i n the i n t e r l a y e r r e g i o n s o f c l a y m i n e r a l s , d i s t i n g u i s h e s them from a d s o r p t i o n r e a c t i o n s t h a t occur at " c o n s t a n t p o t e n t i a l " s u r f a c e s , such as those o f hydrous o x i d e s (8).

In Geochemical Processes at Mineral Surfaces; Davis, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

8

G E O C H E M I C A L P R O C E S S E S AT M I N E R A L S U R F A C E S

W h i l e the t h e o r y and e x p e r i m e n t a l measurements o f the e q u i l i b r i a o f i o n e x c h a n g e a r e w e l l e s t a b l i s h e d ( 6 6 , 6 7 ) , some m o d e l s e m p l o y u n v e r i f i e d a s s u m p t i o n s c o n c e r n i n g t h e s t r u c t u r e o f exchanged i o n s . For example, t h e e x t e n t t o which c a t i o n s are d e h y d r a t e d upon e n t e r i n g an i o n e x c h a n g e r i s n o t known u n e q u i v o c a b l y ( 6 8 ) . S p o s i t o (10) d i s c u s s e s t h e e x p e r i m e n t a l e v i d e n c e f o r f o r m a t i o n o f both i n n e r - and outer-sphere complexes for various exchanging ions. Camontmori 1 1 o n i t e s u s p e n s i o n s e x h i b i t a d(001) s p a c i n g t h a t is c o n s i s t e n t w i t h t h e f o r m a t i o n o f an o u t e r - s p h e r e s u r f a c e c o m p l e x between e x c h a n g e a b l e C a i o n s and a p a i r o f o p p o s i n g s i l o x a n e d i t r i g o n a l c a v i t i e s . A d d i t i o n a l evidence for t h i s s t r u c t u r e includes q u a s i e l a s t i c neutron s c a t t e r i n g experiments which suggest the e x i s t e n c e of a r i g i d o c t a h e d r a l s o l v a t i o n s h e l l for C a ^ i n Camontmori11onitg (69). E l e c t r o n spin resonance spectra for exchangeable C u and M n i n montmori 1 1 o n i t e a l s o suggest the f o r m a t i o n o f o u t e r - s p h e r e c o m p l e x e s ( 7 0 , and s e e M c B r i d e , C h a p t e r 17). Goodman ( C h a p t e r 16) r e v i e w s t h e c u r r e n t s t a t e o f k n o w l e d g e concerning the s o r p t i o n the v a r i o u s s p e c t r o s c o p i c h a r a c t e r i z e the bonding e n v i r o n m e n t . When i s o m o r p h i c s u b s t i t u t i o n o f A l for S i o c c u r s i n the t e t r a h e d r a l s h e e t o f a p h y l 1 o s i 1 i c a t e , t h e e x c e s s n e g a t i ve c h a r g e w i l l d i s t r i b u t e i t s e l f p r i m a r i l y o v e r t h e t h r e e s u r f a c e oxygens o f one t e t r a h e d r o n . T h i s a l l o w s f o r the f o r m a t i o n o f s t r o n g complexes w i t h c a t i o n s ( Π ) ) . In p a r t i c u l a r , t h e f o r m a t i o n o f i n n e r - s p h e r e complexes w i t h K i s l i k e l y , because the i o n i c diameter of K i s a l m o s t e q u a l t o t h e s i z e o f the d i t r i g o n a l c a v i t y i n the b a s a l p l a n e s o f v e r m i c u l i t e and i l l i t i c m i c a s . The i n f l u e n c e o f such s t r u c t u r e s and bonding f o r c e s on i o n exchange p r o c e s s e s i n v o l v i n g K and C a are d i s c u s s e d i n d e t a i l by G o u l d i n g (Chapter 15) and more g e n e r a l l y by Maes and C r e m e r s ( C h a p t e r 13). In b o t h p a p e r s , e v i d e n c e f o r t h e e x i s t e n c e o f h i g h l y s e l e c t i v e and s p e c i f i c s u r f a c e s i t e s i n l a y e r e d m i n e r a l s i s reviewed. Goulding presents a c h a r a c t e r i z a t i o n of several s e l e c t e d l a y e r e d s i l i c a t e m i n e r a l s i n terms o f s p e c i f i c s i t e t y p e s which can be i d e n t i f i e d by t h e e n t h a l p i e s o f i o n exchange. The a u t h o r a l s o d i s c u s s e s reasons f o r t h e s l o w exchange o r " f i x a t i o n " o f K i n s o i l s . In Chapter 13, Maes and Cremers p r e s e n t a comprehensive r e v i e w o f t h e i n f l u e n c e i o n e x c h a n g e r c h a r g e d e n s i t y and t h e r e l a t i v e p o l a r i z a b i 1 i t y o f e x c h a n g i n g i o n s on i o n e x c h a n g e p r o c e s s e s . The f a c t o r s t h a t l e a d t o h i g h l y s e l e c t i v e exchange b e h a v i o r i n montmori 1 l o n i t e s and z e o l i t e s are emphasized. The paper by Yasunaga and Ikeda (Chapter 12) examines the mechanisms o f i n t e r c a l a t i o n and d e i n t e r c a l a t i o n i n l a y e r e d , c h a n n e l e d , and c a g e - s t r u c t u r e d m i n e r a l s . E b e r l e t a l . (Chapter 14) p r e s e n t e v i d e n c e f o r t h e dynamic r o l e o f w e t t i n g and d r y i n g c y c l e s on t h e w e a t h e r i n g o f s m e c t i t e s and Kf e l d s p a r s . The e x c h a n g e o f K w i t h o t h e r c a t i o n s l e a d s t o t h e f o r m a t i o n o f i l l i t e - l i k e l a y e r s i n s m e c t i t e s , but t h i s r e a c t i o n i s c o m p l e t e l y r e v e r s i b l e when e x c h a n g e d a g a i n w i t h c a t i o n s o f h i g h h y d r a t i o n e n e r g y s u c h as C a (71.). H o w e v e r , E b e r l e t a l . show t h a t K - s m e c t i t e may f i x K i r r e v e r s i b l y when s u b j e c t e d t o w e t t i n g and d r y i n g c y c l e s . The m o i s t u r e c o n t e n t o f s o i l s can d r a m a t i c a l l y a f f e c t the r e a c t i v i t y o f s o i l m i n e r a l s . Under v e r y low m o i s t u r e c o n d i t i o n s b o t h t h e s u r f a c e and i n t e r l a y e r a c i d i t y c a n be g r e a t l y i n c r e a s e d , l e a d i n g t o an i n c r e a s e i n t h e r a t e s o f a c i d - c a t a l y z e d r e a c t i o n s . Thus, c h e m i c a l t r a n s f o r m a t i o n s may o c c u r a t q u i t e d i f f e r e n t r a t e s i n 2 +

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In Geochemical Processes at Mineral Surfaces; Davis, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

1.

DAVIS A N D HAYES

9

Overview

u n s a t u r a t e d v s . s a t u r a t e d s o i l z o n e s . In a d d i t i o n t o t h e p a p e r by E b e r l e t a l . , t h i s t o p i c i s a d d r e s s e d i n t h e p a p e r s by V o u d r i a s and R e i n h a r d (Chapter 22) and V e l b e l (Chapter 30). Surface Spectroscopy Surface spectroscopy o f f e r s the best o p p o r t u n i t y to e l u c i d a t e the s t r u c t u r e s o f c h e m i c a l s p e c i e s a t t h e m i n e r a l - w a t e r i n t e r f a c e (see S p o s i t o , C h a p t e r 11). The a p p l i c a t i o n o f s p e c t r o s c o p i c m e t h o d s t o probe the m o l e c u l a r environment of the i n t e r f a c e i s s t i l l a r e l a t i v e l y new f i e l d . C h a p t e r s 16-19 p r e s e n t r e v i e w s and some r e c e n t a d v a n c e s i n i n v e s t i g a t i o n s o f m o l e c u l a r s t r u c t u r e a t the m i n e r a l w a t e r i n t e r f a c e . A r e c e n t r e v i e w o f s p e c t r o s c o p i c methods a p p l i e d t o s o i l and c l a y m i n e r a l systems i s g i v e n i n S t u c k i and Banwart (72). S p e c t r o s c o p i c t e c h n i q u e s can be c l a s s i f i e d a c c o r d i n g t o t h e type of i n t e r f a c i a l environment being i n v e s t i g a t e d . W h i l e the terms " s u r f a c e " and " i n t e r f a c e " are o f t e n i n t e r c h a n g e d , each has a d i s t i n c t meaning: S u r f a c e r e f e r s t gas o r l i q u i d phase; an between the s u r f a c e and a l i q u i d or gas phase. S u r f a c e s p e c t r o s c o p i c t e c h n i q u e s o f t e n r e q u i r e vacuum o r u l t r a h i g h vacuum c o n d i t i o n s and a r e s o m e t i m e s r e f e r r e d t o as e x - s i t u t e c h n i q u e s . S p e c t r o s c o p i c t e c h n i q u e s a p p l i e d t o m i n e r a l - a q u e o u s systems are r e f e r r e d t o as i n s i t u t e c h n i q u e s i n t h a t d i r e c t i n v e s t i g a t i o n o f aqueous s u s p e n s i o n s is possible. S p e c t r o s c o p i c t e c h n i q u e s w h i c h a n a l y z e t h e c o m p o s i t i o n and s t r u c t u r e o f m i n e r a l s u r f a c e s i n c l u d e Auger E l e c t r o n Spectroscopy ( A E S ) , X - r a y P h o t o e l e c t r o n S p e c t r o s c o p y ( X P S , o r t h e o l d e r name, ESCA), and S e c o n d a r y Ion Mass S p e c t r o s c o p y ( S I M S ) . P e r r y ( C h a p t e r 18) d i s c u s s e s t h e a p p l i c a t i o n o f X P S , A E S , and SIMS t o s t u d i e s o f n a t u r a l m a t e r i a l s . Each o f t h e s e t e c h n i q u e s c a n y i e l d d e t a i l e d i n f o r m a t i o n a b o u t t h e s t r u c t u r e and b o n d i n g o f m i n e r a l s and o f c h e m i c a l s p e c i e s p r e s e n t on t h e s u r f a c e s o f m i n e r a l s , b u t P e r r y i l l u s t r a t e s t h e f a c t t h a t a much g r e a t e r knowledge can be g a i n e d by c o m b i n i n g d a t a f r o m two o r more m e t h o d s . The a u t h o r a l s o d i s c u s s e s the i m p o r t a n t s p e c t r a l parameters t h a t y i e l d s t r u c t u r a l i n f o r m a t i o n , t h e a p p l i c a t i o n o f depth p r o f i l i n g methods, and problems c r e a t e d by the h i g h vacuum n e c e s s a r y f o r a n a l y s i s . The use o f XPS and SIMS i n an i n v e s t i g a t i o n o f the o x i d a t i o n s t a t e o f c o b a l t sorbed by b i r n e s s i t e i s r e p o r t e d i n D i l l a r d and Schenck (Chapter 24). A v a r i e t y o f i n - s i t u s p e c t r o s c o p i c t e c h n i q u e s have been used t o i n v e s t i g a t e the m i n e r a l - w a t e r i n t e r f a c e , i n c l u d i n g Raman ( 7 3 ) , F o u r i e r Transform I n f r a r e d (FTIR)(74-75), N u c l e a r M a g n e t i c Resonance (NMR)(76), E l e c t r o n P a r a m a g n e t i c R e s o n a n c e (EPR)(77-80), E l e c t r o n N u c l e a r Double Resonance (END0R)(81), Mossbauer (82), and Extended Xr a y A b s o r p t i o n F i n e S t r u c t u r e (EXAFS)(83) s p e c t r o s c o p i e s . I n ^ s r t u s p e c t r o s c o p i c i n v e s t i g a t i o n s o f t h e m i n e r a l - w a t e r i n t e r f a c e can be f o u n d i n t h e p a p e r s by Z e l t n e r e t a l . ( F T I R , C h a p t e r 8 ) , M c B r i d e (EPR, Chapter 17) and Ambe e t a l . (Mossbauer, C h a p t e r 19). M c B r i d e (Chapter 17) s t u d i e d the o r i e n t a t i o n and m o b i l i t y o f C u i o n s sorbed at exchange s i t e s o f l a y e r s i l i c a t e m i n e r a l s . The EPR s p e c t r a l d a t a r e v e a l e d that the r o t a t i o n a l motion of C u i n t h e s e m i n e r a l s was h i g h l y dependent on the s i z e o f t h e i n t e r l a y e r r e g i o n , and hence, the degree o f i n t e r l a y e r e x p a n s i o n . Ambe e t a l . (Chapter 19) d i s c u s s the s t r u c t u r e s o f s o r b e d C o ( I I ) and Sb(V) i o n s on t h e s u r f a c e o f z +

z +

In Geochemical Processes at Mineral Surfaces; Davis, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

10

G E O C H E M I C A L P R O C E S S E S AT M I N E R A L S U R F A C E S

h e m a t i t e . An i n - s i t u e m i s s i o n Mossbauer s p e c t r o s c o p i c s t u d y o f t h e s e s u r f a c e s r e v e a l e d two c h e m i c a l f o r m s o f a d s o r b e d C o ( I I ) : one a t t r i b u t a b l e t o c o o r d i n a t i v e bonds t o s u r f a c e s i t e s and t h e o t h e r to w e a k l y b o u n d ( p o s s i b l y h y d r o g e n b o n d e d ) i o n s . The relative p r o p o r t i o n s o f the two forms v a r i e d as a f u n c t i o n o f pH. Two forms o f a d s o r b e d Sb(V) were a l s o f o u n d i n t h i s s t u d y . Goodman ( C h a p t e r 16) d i s c u s s e s t h e use o f EPR and o t h e r s p e c t r o s c o p i c t e c h n i q u e s i n s t u d i e s of the a d s o r p t i o n of metal i o n s a n d c o m p l e x e s by aluminosilicate minerals. Many o t h e r methods c a n be u s e d t o o b t a i n i n f o r m a t i o n a b o u t m i n e r a l s u r f a c e s i n c l u d i n g Low E n e r g y E l e c t r o n D i f f r a c t i o n (LEED), S c a n n i n g E l e c t r o n M i c r o s c o p y (SEM) and T r a n s m i s s i o n E l e c t r o n M i c r o s c o p y (TEM) (72). The d i f f r a c t i o n methods are w e l l e s t a b l i s h e d f o r s t u d y i n g s t r u c t u r e and the m i c r o s c o p i c t e c h n i q u e s can r e v e a l the morphology o f s u r f a c e s . G i e s e and Constanzo (Chapter 3) s t u d i e d the bonding of i n t e r c a l a t e d water m o l e c u l e s i n s y n t h e t i c hydrated k a o l i n i t e by i n f r a r e d (IR) a b s o r p t i o n . V o u d r i a s and R e i n h a r d (Chapter 22) r e v i e w the a p p l i c a t i o for i d e n t i f y i n g sorbed o r g a n i c r e a c t i o n s at the m i n e r a l - w a t e r i n t e r f a c e . D i s s o l u t i o n , P r e c i p i t a t i o n , and S o l i d S o l u t i o n F o r m a t i o n D i s s o l u t i o n . C h e m i c a l w e a t h e r i n g i s c e r t a i n l y among t h e most i m p o r t a n t geochemical p r o c e s s e s t h a t o c c u r on t h e e a r t h ' s s u r f a c e . In t h e l a s t decade, r e s e a r c h i n t h i s f i e l d has focused on t h e k i n e t i c s and m e c h a n i s m s o f m i n e r a l d i s s o l u t i o n r e a c t i o n s , s i n c e a d e p a r t u r e from e q u i l i b r i u m i n n a t u r a l systems i s apparent (84-85). Hypotheses f o r d i s s o l u t i o n m e c h a n i s m s and r a t e - l i m i t i n g s t e p s d u r i n g t h e w e a t h e r i n g p r o c e s s c a n be g r o u p e d i n t o two s c h o o l s o f t h o u g h t (86)· One s c h o o l proposes t h a t the d i s s o l u t i o n r a t e i s c o n t r o l l e d by the f o r m a t i o n o f a r e s i d u a l l a y e r at the s u r f a c e o f t h e r e a c t i n g m i n e r a l , t h r o u g h which t h e r e a c t a n t s and p r o d u c t s o f w e a t h e r i n g must d i f f u s e ( 8 7 , 8 8 ) . The s e c o n d g r o u p p r o p o s e s t h a t the d i s s o l u t i o n r a t e i s c o n t r o l l e d by the r a t e o f a s u r f a c e r e a c t i o n (89,90). The proponents o f t h e d i f f u s i o n - c o n t r o l m e c h a n i s m b a s e t h e i r c o n c l u s i o n s on t h e temporal e v o l u t i o n o f aqueous s o l u t i o n c o m p o s i t i o n , which suggests t h a t t h e k i n e t i c s o f m i n e r a l d i s s o l u t i o n u s u a l l y obeys a p a r a b o l i c r a t e law. E v i d e n c e f o r the l a t t e r h y p o t h e s i s i n c l u d e s the r e s u l t s o f s u r f a c e s p e c t r o s c o p i c s t u d i e s which have f a i l e d t o d e t e c t any l e a c h e d l a y e r (89-91). D i b b l e and T i l l e r (28) noted t h a t the c o n s i s t e n c y o f the k i n e t i c d a t a w i t h p a r a b o l i c r a t e l a w may i n d i c a t e t h a t t h e r a t e determining step i n v o l v e s d i f f u s i o n of a r e a c t i o n product or impurity i o n t o o r from the i n t e r f a c e . W h i l e mass t r a n s f e r through t h e l i q u i d phase at the i n t e r f a c e i s a r e l a t i v e l y f a s t s t e p , d i f f u s i o n c o u l d become r a t e - d e t e r m i n i n g i f a d s o r p t i o n r e t a r d s m o l e c u l a r detachments a t k i n k s o r l a y e r e d g e s (92). V e l b e l ( C h a p t e r 30) r e v i e w s t h i s a r e a o f r e s e a r c h and d i s c u s s e s c u r r e n t a p p l i c a t i o n s o f t h e c o n c l u s i o n s to n a t u r a l systems. A common phenomenon i n the d i s s o l u t i o n o f s i l i c a t e m i n e r a l s i s t h e f o r m a t i o n o f e t c h p i t s a t t h e s u r f a c e ( 9 0 - 9 3 . , 9 3 - 9 4 ) . When t h i s o c c u r s , the o v e r a l l r a t e o f m i n e r a l d i s s o l u t i o n i s n o n - u n i f o r m , and d i s s o l u t i o n o c c u r s p r e f e r e n t i a l l y at d i s l o c a t i o n s or defects that i n t e r c e p t the c r y s t a l s u r f a c e . P r e f e r e n t i a l d i s s o l u t i o n of the m i n e r a l c o u l d e x p l a i n why s u r f a c e s p e c t r o s c o p i c s t u d i e s have f a i l e d

In Geochemical Processes at Mineral Surfaces; Davis, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

1.

DAVIS A N D H A Y E S

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t o d e t e c t a l e a c h e d l a y e r a t t h e s u r f a c e (93). B r a n t l e y e t a l . ( C h a p t e r 31) e x a m i n e t h e f o r m a t i o n o f e t c h p i t s on q u a r t z i n l a b o r a t o r y e x p e r i m e n t s a t 300°C and on q u a r t z p a r t i c l e s f r o m a n a t u r a l s o i l p r o f i l e . T h e i r r e s u l t s suggest t h a t t h e t h e o r y o f e t c h p i t f o r m a t i o n a t d i s l o c a t i o n s may be u s e f u l i n d e s c r i b i n g m i n e r a l w e a t h e r i n g b e h a v i o r a t l o w t e m p e r a t u r e s as w e l l as d u r i n g hydrothermal a l t e r a t i o n . Numerous g e o c h e m i c a l s t u d i e s h a v e a t t e m p t e d t o i n t e r p r e t t h e c o m p o s i t i o n o f waters i n terms o f c h e m i c a l r e a c t i o n s between parent m i n e r a l s and w e a t h e r i n g p r o d u c t s i n n e a r - s u r f a c e w e a t h e r i n g e n v i r o n m e n t s . The s t u d i e s s u g g e s t , or i n some cases assume, t h a t the water i s i n e q u i l i b r i u m w i t h e i t h e r o b s e r v e d o r i n f e r r e d w e a t h e r i n g p r o d u c t s o r h y p o t h e t i c a l m e t a s t a b l e phases (95). However, c h e m i c a l e q u i l i b r i u m i s n o t n e c e s s a r i l y e x p e c t e d i n an open s y s t e m t h r o u g h w h i c h w a t e r i s f l u x i n g r a p i d l y . A more r e a l i s t i c a p p r o a c h i s t o r e l a t e t h e w a t e r c h e m i s t r y t o t h e k i n e t i c s o f d i s s o l u t i o n and p r e c i p i t a t i o n o f p r i m a r y and s e c o n d a r y phases (87,96). Weathering r a t e s i n n a t u r e are u s u a l l e q u a t i o n s (97). V e l b e l ( C h a p t e m i n e r a l w e a t h e r i n g r a t e s measured i n n a t u r e which can be n o r m a l i z e d on t h e b a s i s o f s u r f a c e a r e a w i t h t h o s e m e a s u r e d i n t h e l a b o r a t o r y (28,98). The r e s u l t s i n d i c a t e t h a t the r a t e s o f d i s s o l u t i o n i n n a t u r e are much s l o w e r than p r e d i c t e d from l a b o r a t o r y e x p e r i m e n t s . V e l b e l o u t l i n e s p o s s i b l e reasons f o r t h i s d i s c r e p a n c y and p r e s e n t s areas i n which f u r t h e r r e s e a r c h are needed. P r e c i p i t a t i o n . An i m p o r t a n t element o f any geochemical a n a l y s i s o f n a t u r a l waters i s an e v a l u a t i o n o f which m i n e r a l s a r e p r e s e n t and t h e e x t e n t t o w h i c h t h e s y s t e m c a n be r e p r e s e n t e d by e q u i l i b r i u m m o d e l s . T y p i c a l q u e s t i o n s t h a t need t o be a n s w e r e d a r e : 1) I s t h e water s u p e r s a t u r a t e d , u n d e r s a t u r a t e d , o r a t e q u i l i b r i u m w i t h a g i v e n m i n e r a l ? and 2) I f more t h a n one s o l i d p h a s e c a n f o r m f o r a g i v e n e l e m e n t , which phase i s more s t a b l e i n t h a t p a r t i c u l a r e n v i r o n m e n t ? P r e c i p i t a t i o n can o c c u r i f a w a t e r i s s u p e r s a t u r a t e d w i t h respect to a s o l i d phase; however, if the growth of a t h e r m o d y n a m i c a l l y s t a b l e phase i s s l o w , a m e t a s t a b l e phase may form. D i s o r d e r e d , amorphous p h a s e s s u c h as f e r r i c h y d r o x i d e , a l u m i n u m h y d r o x i d e , and a l l o p h a n e are thermodynamical 1 y u n s t a b l e w i t h r e s p e c t to c r y s t a l l i n e phases; n o n e t h e l e s s , these d i s o r d e r e d phases are f r e q u e n t l y f o u n d i n n a t u r e . The r a t e s o f c r y s t a l l i z a t i o n o f t h e s e phases are s t r o n g l y c o n t r o l l e d by t h e presence o f adsorbed i o n s on t h e s u r f a c e s o f p r e c i p i t a t e s ( 9 9 ) . Z a w a c k i e t a l . ( C h a p t e r 32) present evidence that adsorption of a l k a l i n e earth ions g r e a t l y i n f l u e n c e s t h e f o r m a t i o n and g r o w t h o f c a l c i u m p h o s p h a t e s . W h i l e h y d r o x y a p a t i t e was t h e t h e r m o d y n a m i c a l l y s t a b l e p h a s e u n d e r t h e c o n d i t i o n s s t u d i e d by t h e s e a u t h o r s , i t i s shown t h a t s e v e r a l d i f f e r e n t m e t a s t a b l e p h a s e s may f o r m , d e p e n d i n g upon t h e d e g r e e o f s u p e r s a t u r a t i o n and the i n i t i a t i n g s u r f a c e phase. P r e c i p i t a t i o n must b e g i n w i t h t h e f o r m a t i o n o f n u c l e i ; n u c l e a t i o n can be homogeneous (formed i n t h e aqueous s o l u t i o n by the spontaneous a s s o c i a t i o n o f i o n s ) o r heterogeneous ( o r i g i n a t i n g on the s u r f a c e o f an i m p u r i t y o r v i a s e e d p a r t i c l e s w h i c h a c t as c r y s t a l l i z a t i o n c a t a l y s t s ) . In n a t u r e , i t i s t h o u g h t that heterogeneous n u c l e a t i o n i s the predominant process which begins p r e c i p i t a t i o n ( 1 0 0 ) . The f a c t o r s d e t e r m i n i n g growth k i n e t i c s may be d i v i d e d i n t o two main groups: 1) t r a n s p o r t p r o c e s s e s the t r a n s p o r t

In Geochemical Processes at Mineral Surfaces; Davis, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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G E O C H E M I C A L P R O C E S S E S AT M I N E R A L S U R F A C E S

o f r e a c t a n t s up t o a c r y s t a l s u r f a c e and t h e t r a n s p o r t o f r e a c t i o n p r o d u c t s away f r o m t h e s u r f a c e , and 2) s u r f a c e p r o c e s s e s , w h i c h may i n c l u d e a d s o r p t i o n o f m o l e c u l e s and i o n s on a c r y s t a l surface, m i g r a t i o n o f i o n s a l o n g the s u r f a c e , a change o f t h e degree o f h y d r a t i o n o f i o n s , f o r m a t i o n o f two d i m e n s i o n a l s u r f a c e n u c l e i , and t h e f i t t i n g o f i o n s i n t o the c r y s t a l l a t t i c e (101). N e i l sen (Chapter 29) r e v i e w s some o f the i m p o r t a n t a s p e c t s o f c r y s t a l growth p r o c e s s e s and t h e f a c t o r s w h i c h i n f l u e n c e t h e r a t e - d e t e r m i n i n g mechanism f o r c r y s t a l growth. A d s o r p t i o n may i n f l u e n c e p r e c i p i t a t i o n by means o t h e r than the p r o c e s s e s mentioned above. D a v i e s (Chapter 23) d i s c u s s e s t h e r o l e o f t h e s u r f a c e as a c a t a l y s t f o r o x i d a t i o n o f a d s o r b e d Mn . Redox r e a c t i o n s may c o n t r i b u t e s u b s t a n t i a l l y t o the f o r m a t i o n o f manganese o x i d e c o a t i n g s on m i n e r a l s u r f a c e s i n s o i l s and sediments. Sol i d Sol u t i o n s . The aqueous c o n c e n t r a t i o n s o f t r a c e e l e m e n t s i n n a t u r a l waters are f r e q u e n t l y much l o w e r than w o u l d be expected on the b a s i s o f e q u i l i b r i u m s o l u b i l i t y c a l c u l a t i o n s o r o f s u p p l y t o the water from v a r i o u s s o u r c e s t h e element on m i n e r a l s u r f a c e c o n c e n t r a t i o n o f t h e t r a c e e l e m e n t (97). However, S p o s i t o ( C h a p t e r 11) shows t h a t t h e methods commonly u s e d t o d i s t i n g u i s h between s o l u b i l i t y o r a d s o r p t i o n c o n t r o l s are c o n c e p t u a l l y f l a w e d . One o f the i m p o r t a n t problems i l l u s t r a t e d i n Chapter 11 i s t h e e v a l u a t i o n o f the s t a t e o f s a t u r a t i o n o f n a t u r a l waters w i t h r e s p e c t t o s o l i d phases. G e n e r a l l y , t h e c o n c l u s i o n t h a t a t r a c e element i s u n d e r s a t u r a t e d i s based on a comparison o f i o n a c t i v i t y p r o d u c t s w i t h known pure s o l i d phases t h a t c o n t a i n the t r a c e element. I f a s o l i d phase i s p u r e , then i t s a c t i v i t y i s equal t o one by thermodynamic c o n v e n t i o n . However, when a t r a c e c a t i o n i s c o p r e c i p i t a t e d w i t h a n o t h e r c a t i o n , t h e a c t i v i t y o f t h e s o l i d phase end member c o n t a i n i n g t h e t r a c e c a t i o n i n t h e c o p r e c i p i t a t e w i l l be l e s s t h a n one. I f t h e a q u e o u s p h a s e i s a t equi 1 ibriurn w i t h the c o p r e c i p i t a t e , then the ion a c t i v i t y product wi 1 1 be 1 e s s t h a n t h e s o l u b i 1 i t y c o n s t a n t o f t h e p u r e s o l i d p h a s e c o n t a i n i n g t h e t r a c e element. T h i s c o n d i t i o n c o u l d then l e a d t o the c o n c l u s i o n t h a t a n a t u r a l water was u n d e r s a t u r a t e d w i t h r e s p e c t to t h e pure s o l i d phase and t h a t the aqueous c o n c e n t r a t i o n o f the t r a c e c a t i o n was c o n t r o l l e d by a d s o r p t i o n on m i n e r a l s u r f a c e s . W h i l e t h i s m i g h t be t r u e , S p o s i t o p o i n t s o u t t h a t t h e i o n a c t i v i t y p r o d u c t comparison w i t h the s o l u b i l i t y p r o d u c t does not p r o v i d e any c o n c l u s i v e e v i d e n c e as t o whether an a d s o r p t i o n o r c o p r e c i p i t a t i o n p r o c e s s c o n t r o l s the aqueous c o n c e n t r a t i o n . T h e r e i s c o n s i d e r a b l e e v i d e n c e t h a t c o p r e c i p i t a t i o n and t h e f o r m a t i o n o f s o l i d s o l u t i o n s i s s i g n i f i c a n t i n s o i l s and sediments ( 1 0 ) . W h i l e i d e a l s o l u t i o n m o d e l s h a v e been w i d e l y p r o p o s e d f o r v a r i o u s m i n e r a l s o l i d s o l u t i o n s , e x p e r i m e n t a l i n v e s t i g a t i o n s and s t u d i e s o f n a t u r a l m i n e r a l assemblages have shown t h a t m i s c i b i l i t y g a p s a r e common i n a l m o s t e v e r y m a j o r m i n e r a l g r o u p ( 1 0 2 ) . The e x i s t e n c e o f such gaps r e q u i r e s n o n i d e a l s o l u t i o n models t o d e s c r i b e t h e d i s t r i b u t i o n o f c o m p o n e n t s i n t h e s o l i d and a q u e o u s p h a s e s . D r i e s s e n s ( C h a p t e r 25) r e v i e w s t h e i m p o r t a n t l i t e r a t u r e c o n c e r n i n g l a b o r a t o r y i n v e s t i g a t i o n s o f s o l i d s o l u t i o n s and p r e s e n t s t h e t h e o r y o f i d e a l s o l i d s o l u t i o n s and n o n i d e a l s o l i d s o l u t i o n s , i n c l u d i n g the more g e n e r a l m o d e l s o f r e g u l a r s o l i d s o l u t i o n s w i t h and w i t h o u t o r d e r i n g . Many o f the c a l c u l a t e d a c t i v i t y c o e f f i c i e n t s f o r end-member s o l i d p h a s e s i n s o l i d s o l u t i o n s a r e b a s e d on an a s s u m p t i o n t h a t a

In Geochemical Processes at Mineral Surfaces; Davis, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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Overview

system has reached e q u i l i b r i u m when the s o l i d phase c o m p o s i t i o n i s m e a s u r e d . Plummer ( C h a p t e r 26) e x a m i n e s t h e c r i t e r i a f o r d e c i d i n g when a s y s t e m has r e a c h e d e q u i l i b r i u m ; h i s a n a l y s i s shows t h a t p r e v i o u s c a l c u l a t i o n s f o r the K C l - K B r system at 25°C may be i n e r r o r because the system a n a l y z e d was o n l y near e q u i l i b r i u m . N o n - l a t t i c e s i t e s may p l a y an i m p o r t a n t r o l e i n the i n c o r p o r a t i o n of l a r g e foreign ions in c r y s t a l s t r u c t u r e s during c o p r e c i p i t a t i o n ; P i n g i t o r e (Chapter 27) d i s c u s s e s the importance o f t h e s e s i t e s i n the formation of c o p r e c i p i t a t e s of c a l c i u m carbonate c o n t a i n i n g S r or Ba . W h i t e and Yee ( C h a p t e r 28) d i s c u s s the d i f f u s i o n of a l k a l i i o n s i n t o d e f e c t s t r u c t u r e s i n t h e s u r f a c e s o f g l a s s e s and c r y s t a l l i n e feldspars. z +

Transformation Reactions at the Mineral/Water Interface C e r t a i n p r o p e r t i e s of the m i n e r a l - w a t e r i n t e r f a c e or of r e a c t i v e s i t e s on m i n e r a l s u r f a c e s may l o w e r the a c t i v a t i o n energy o f v a r i o u s transformation reactions h y d r o l y s i s reactions of o r L e w i s a c i d s i t e s on m i n e r a l s u r f a c e s i s a p r i m a r y f a c t o r i n c a t a l y z i n g such r e a c t i o n s at the s u r f a c e . Other f a c t o r s are the s t r u c t u r e and charge o f the m i n e r a l s u r f a c e ( i n c l u d i n g any i n t e r l a y e r s p a c i n g ) and t h e s i z e and charge o f r e a c t i n g s o l u t e s . I t i s g e n e r a l l y b e l i e v e d t h a t t h e m e c h a n i s m s o f t h e s e t r a n s f o r m a t i o n r e a c t i o n s are s i m i l a r t o those t h a t o c c u r i n homogeneous aqueous s o l u t i o n s , but the c o n d i t i o n s at t h e m i n e r a l - w a t e r i n t e r f a c e may a c c e l e r a t e the r a t e s o f c e r t a i n r e a c t i o n s . A l t h o u g h many i m p o r t a n t r e a c t i o n s o f o r g a n i c compounds a r e c a t a l y z e d by m i n e r a l s u r f a c e s u n d e r d e s s i c a t e d c o n d i t i o n s at e l e v a t e d temperatures (103), l i t t l e i n f o r m a t i o n i s a v a i l a b l e on c a t a l y s i s by s u r f a c e s i n aqueous e n v i r o n m e n t s . Electron Transfer Reactions. Important redox r e a c t i o n s i n v o l v i n g aqueous s o l u t e s and m i n e r a l s u r f a c e s i n c l u d e o x i d a t i v e o r r e d u c t i v e d i s s o l u t i o n , o x i d a t i o n o r r e d u c t i o n o f s o l u t e s by r e a c t i o n w i t h s u r f a c e s i t e s , and p o l y m e r i z a t i o n r e a c t i o n s o f o r g a n i c compounds. The theory of e l e c t r o n t r a n s f e r r e a c t i o n s i s w e l l e s t a b l i s h e d i n homogeneous s o l u t i o n ; however, the mechanisms o f e l e c t r o n t r a n s f e r r e a c t i o n s w h i c h o c c u r a t t h e m i n e r a l - w a t e r i n t e r f a c e a r e more d i f f i c u l t t o e s t a b l i s h because o f the d i f f i c u l t y i n i d e n t i f y i n g the reacting species. Two t y p e s o f e l e c t r o n t r a n s f e r m e c h a n i s m s h a v e been f o u n d f r o m k i n e t i c s t u d i e s i n homogeneous s o l u t i o n : 1) i n n e r sphere and 2) o u t e r - s p h e r e . S t o n e ( C h a p t e r 21) d i s c u s s e s the a n a l o g y b e t w e e n e l e c t r o n t r a n s f e r r e a c t i o n s i n homogeneous and heterogeneous systems. Waite (Chapter 20) r e v i e w s t h e l i t e r a t u r e on the a b i l i t y o f l i g h t to i n i t i a t e or enhance t h e r a t e s o f redox r e a c t i o n s w h i c h o c c u r on m i n e r a l s u r f a c e s . A s t u d y o f a c c e l e r a t e d oxidation of M n i n the presence o f hydrous i r o n o x i d e i s p r e s e n t e d by D a v i e s ( C h a p t e r 2 3 ) . D i l l a r d and S c h e n c k ( C h a p t e r 24) r e p o r t on r e d o x r e a c t i o n s o f C o ( I I ) and C o ( I I I ) - c o m p l e x e s on t h e s u r f a c e o f birnessite. The s u r f a c e s o f c l a y m i n e r a l s can c a t a l y z e t h e p o l y m e r i z a t i o n o f o r g a n i c compounds through a f r e e r a d i c a l - c a t i o n i c i n i t i a t i o n p r o c e s s . T h i s t y p e o f r e a c t i o n i s b e l i e v e d t o be i n i t i a t e d by t h e a b s t r a c t i o n o f an e l e c t r o n by L e w i s a c i d s i t e s on m i n e r a l s u r f a c e s ; h o w e v e r , B r o n s t e d a c i d i t y has a l s o been shown t o be i m p o r t a n t i n c e r t a i n cases (see C h a p t e r 2 2 ) . z +

In Geochemical Processes at Mineral Surfaces; Davis, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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GEOCHEMICAL PROCESSES ATMINERAL SURFACES

A d d i t i o n a l Transformation Reactions. Other r e a c t i o n s t h a t can be c a t a l y z e d by m i n e r a l s u r f a c e s a r e s u b s t i t u t i o n , e l i m i n a t i o n , and a d d i t i o n r e a c t i o n s o f o r g a n i c m o l e c u l e s . S u b s t i t u t i o n and e l i m i n a t i o n a r e two g e n e r a l t y p e s o f r e a c t i o n s t h a t o c c u r a t s a t u r a t e d c a r b o n atoms o f o r g a n i c m o l e c u l e s . Both types a r e i n i t i a t e d by n u c l e o p h i l i c a t t a c k ; however, i n e l i m i n a t i o n r e a c t i o n s i t i s t h e b a s i c i t y o f t h e n u c l e o p h i l e that determine i t s r e a c t i v i t y r a t h e r than i t s nucleophilicity. S i n c e m i n e r a l s u r f a c e s a r e expected t o have both n u c l e o p h i l i c and b a s i c p r o p e r t i e s , t h e s e t y p e s o f r e a c t i o n s s h o u l d a l s o o c c u r a t m i n e r a l - w a t e r i n t e r f a c e s (see C h a p t e r 22). I t remains t o be shown w h e t h e r o r n o t t h e s e r e a c t i o n s a r e c a t a l y z e d u n d e r environmental conditions. H y d r o l y s i s r e a c t i o n s o c c u r by n u c l e o p h i l i c a t t a c k a t a carbon s i n g l e bond, i n v o l v i n g e i t h e r t h e water m o l e c u l e d i r e c t l y o r t h e h y d r o n i u m o r h y d r o x y l i o n . The most f a v o r a b l e c o n d i t i o n s f o r h y d r o l y s i s , e.g. a c i d i c o r a l k a l i n e s o l u t i o n s , depend on t h e n a t u r e o f t h e bond w h i c h i s t o be c l e a v e d . M i n e r a l s u r f a c e s t h a t have B r o n s t e d a c i d i t y have bee Examples o f h y d r o l y s i s u r f a c e s o f m i n e r a l s i n s o i l s i n c l u d e p e p t i d e bond f o r m a t i o n by amino a c i d s which a r e adsorbed on c l a y m i n e r a l s u r f a c e s and t h e d e g r a d a t i o n of p e s t i c i d e s (see Chapter 22). C o n c l u d i n g Remarks Our k n o w l e d g e o f t h e p h y s i c a l and c h e m i c a l n a t u r e o f t h e m i n e r a l water i n t e r f a c e i s s t i l l a d v a n c i n g . The s i g n i f i c a n c e o f t h e i n t e r f a c e i n p r o c e s s e s s u c h as s o r p t i o n , i o n e x c h a n g e , p r e c i p i t a t i o n , and d i s s o l u t i o n h a s been r e c o g n i z e d f o r some t i m e , b u t new s t u d i e s a r e d e m o n s t r a t i n g t h a t t h e s e p r o c e s s e s a r e i n t e r r e l a t e d i n complex and i n t e r e s t i n g ways. A f u l l a p p r e c i a t i o n o f t h e fundamental importance o f i n t e r f a c i a l r e a c t i o n s i n geochemical p r o c e s s e s i s s t i l l emerging, and i t i s i n c r e a s i n g l y c l e a r t h a t t h e i n t e r f a c e may p l a y a c r i t i c a l r o l e i n a c c e l e r a t i n g t h e r a t e s o f redox r e a c t i o n s , p o l y m e r i z a t i o n , h y d r o l y s i s , and o t h e r t r a n s f o r m a t i o n s . T h i s v o l u m e p r e s e n t s a compilation of state-of-the-art theoretical and e x p e r i m e n t a l approaches which a r e b e i n g a p p l i e d i n s t u d i e s o f t h e m i n e r a l - w a t e r i n t e r f a c e . T h e s e new c o n c e p t s must be i n t e g r a t e d i n t o g e o c h e m i c a l models i f a comprehensive c h e m i c a l d e s c r i p t i o n o f n a t u r a l systems i s t o be a c h i e v e d . Acknowledgments The a u t h o r s w o u l d l i k e t o thank A. Maest, C. C h i s h o l m , and C. F u l l e r for t h e i r c r i t i c i a l reviews o f the manuscript.

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1. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23 24. 25. 26. 27. 28. 29. 30.

DAVIS AND HAYES Overview

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61. Honeyman, B.D. Ph.D. Thesis, Stanford University, Stanford, Calif., 1984. 62. Tessier, Α.; Rapin, F.; Carignan, R. Geochim. Cosmochim. Acta 1985, 49, 183-194. 63. Balistrieri, L.S.; Murray, J.W. Geochim. Cosmochim. Acta 1983, 47, 1091-1098. 64. Karickhoff, S.W. J. Hydraulic Eng. 1984, 110, 707-735. 65. Sposito, G.; Mattigod, S.V. Son Sci. Soc. Am. J. 1977, 41, 323329. 66. Sposito, G. "Thermodynamics of Soil Solutions"; Oxford Clarendon Press: Oxford, 1981. 67. Bruggenwert, M.G.M.; Kamphorst, A. In "Soil Chemistry, B. Physico-Chemical Models" Bolt, G.H. Ed.; Elsevier Sci. Publ.: Amsterdam, 1979, Chap. 5. 68. Maes, Α.; Cremers, A. In "Soil Chemistry B. Physico-Chemical Models" Bolt, G.H. Ed.; Elsevier Sci. Publ.: Amsterdam, 1979, Chap. 6. 69. Ross, D.K.; Hall, P.L Clay Mineralogy Research" Reidel: Boston, 1980, p. 93. 70. McBride, M.B.; Pinnavaia, T.J.; Mortland, M.M. J. Phys. Chem. 1973, 77, 196-200. 71. Goulding, K.W.T.; Talibudeen, O. J. Colloid Interface Sci. 1980, 78, 15-24. 72. Stucki, J.W.; Banwart, W.L., Eds. "Advanced Chemical Methods for Soil and Clay Minerals Research"; Reidel: Boston, 1980. 73. Johnston, C.T.; Sposito, G.; Birge, R.R. Clays andC l a yMin. 1985, 33, 483-489. 74. Tejedor-Tejedor, M.I.; Anderson, M.A. Langmuir 1986, 2, 203-210. 75. Foley, J.K.; Pons, S. Anal. Chem. 1985, 57, 945A-956A. 76. Young, J.R. PhD. Thesis, California Institute of Technology, Pasadena, Calif., 1981. 77. Motschi, H. Colloids and Surfaces 1984, 9, 337-347. 78. Fransesca, M.O.; Ceresa, E.M.; Visca, M. J. Colloid Interface Sci. 1985, 108, 114-122. 79. Bassetti, V.; Burlamacchi, L.; Martini, G. J. Amer. Chem. Soc. 1979, 101, 5471-5477. 80. Clementz, D.M.; Pinnavaia, T.J.; Mortland, M.M. J. Phys. Chem. 1973, 77, 196-200. 81. Rudin, M.; Motschi, H. J. Colloid Interface Sci. 1984, 98, 385393. 82. Ambe, F.; Okada, T.; Ambe, S.; Sekizawa, H. J. Phys. Chem. 1984, 88, 3015. 83. Waychunas, G.A.; Brown, G.E. In EXAFS and Near Edge Structure III; Hodgson, K.O.; Hedman, B.; Penner-Hahn, J . E . , Eds.; Springer-Verlag, New York, pp. 336-342. 84. Berner, R.A. Aim J. Sci. 1978, 278, 1235-1252. 85. Lerman, Α., "Geochemical Processes"; Wiley: New York, 1979. 86. Wollast, R.; Chou, L. In "The Chemistry of Weathering"; Drever, J.I., Ed.; Reidel: Boston, 1985, pp. 75-96. 87. Paces, T. Geochim. Cosmochim. Acta 1973, 37, 2641-2663. 88. Wollast, R. Geochim. Cosmochim. Acta 1967, 31, 635-648. 89. Holdren, G.R.; Berner, R.A. Geochim. Cosmochim. Acta 1979, 43, 1161-1171.

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In Geochemical Processes at Mineral Surfaces; Davis, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

2

S i m u l a t i n g L i q u i d Water near M i n e r a l Surfaces: Current Methods and Limitations David J. Mulla Department of Agronomy and Soils, Washington State University, Pullman, WA 99164-6420 It is important to propose molecular and theoretical models to describ th forces structur d dynamics of water understanding of experimenta g hydration forces, the hydrophobic effect, swelling, reaction kinetics and adsorption mechanisms in aqueous colloidal systems is rapidly advancing as a result of recent Monte Carlo (MC) and molecular dynamics (MD) models for water properties near model surfaces. This paper reviews the basic MC and MD simulation techniques, compares and contrasts the merits and limitations of various models for water-water interactions and surfacewater interactions, and proposes an interaction potential model which would be useful in simulating water near hydrophilic surfaces. In addition, results from selected MC and MD simulations of water near hydrophobic surfaces are discussed in relation to experimental results, to theories of the double layer, and to structural forces in interfacial systems. Recent evidence (1) suggests that reactions at the mineral/liquid interface were involved in the beginnings of life on Earth. Not surprisingly, the nature and properties of mineral/water interfaces are of interest to physicists, chemists, physical chemists, applied mathematicians, colloid scientists, geochemists, soil scientists and civil engineers. Of particular interest is an increased understanding of the role of water in colloidal swelling, solute hydration, reaction kinetics, adsorption mechanisms, and ion exchange. The theoretical study (2,3) of this interface is made inherently difficult by virtue of the complex, many-body nature of the interaction potentials and forces involving surfaces, counterions, and water. Hence, many models of the interfacial region explicitly specify the forces between colloidal particles or between solutes, but few account for the many-body interaction forces of the solvent. 0097-6156/ 86/ 0323-0020S06.00/ 0 © 1986 American Chemical Society

In Geochemical Processes at Mineral Surfaces; Davis, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

2.

MULLA

Simulating

Liquid

Water near Mineral

Surfaces

21

E x p e r i m e n t a l s t u d i e s o f the thermodynamic, s p e c t r o s c o p i c and t r a n s p o r t p r o p e r t i e s o f m i n e r a l / w a t e r i n t e r f a c e s have been e x t e n s i v e , a l b e i t c o n f l i c t i n g a t times ( 4 - 1 0 ) . Ambiguous terms such as " h y d r a t i o n f o r c e s " , " h y d r o p h o b i c i n t e r a c t i o n s " , and " s t r u c t u r e d w a t e r " have a r i s e n to d e s c r i b e i n t e r f a c i a l p r o p e r t i e s which have been d i f f i c u l t to q u a n t i f y and e x p l a i n . A d e t a i l e d s t a t i s t i c a l - m e c h a n i c a l d e s c r i p t i o n of the f o r c e s , e n e r g i e s and p r o p e r t i e s of water a t m i n e r a l surfaces i s c l e a r l y d e s i r a b l e . M o l e c u l a r p r e d i c t i o n s of the p r o p e r t i e s o f i n t e r f a c i a l systems are now becoming p o s s i b l e as a r e s u l t o f r a p i d advances i n l i q u i d s t a t e chemical p h y s i c s and computer t e c h n o l o g y . The o b j e c t i v e s of t h i s paper are 1) to r e v i e w the general approaches and models used i n Monte C a r l o (MC) and m o l e c u l a r dynamics (MD) s i m u l a t i o n s of i n t e r f a c i a l s y s t e m s , 2) to d e s c r i b e and d i s c u s s r e s u l t s from s e l e c t e d s i m u l a t i o n s t u d i e s of i n t e r f a c i a l w a t e r , and 3) to d i s c u s s the major l i m i t a t i o n s of these t e c h n i q u e s and to o f f e r s u g g e s t i o n s f o r overcoming them. General S i m u l a t i o n Approaches In most MC (11,12) and MD (12,13) p a r t i c l e s are p l a c e d i n a c e l l of i n t e r a c t i o n p o t e n t i a l energy (U ) p o t e n t i a l s (U-jj) between p a r t i c l e s N

u

N

=

S

I

s t u d i e s , a s m a l l number (N) of f i x e d volume (V) and the t o t a l from a l l p a i r w i s e i n t e r a c t i o n i and j i s c a l c u l a t e d :

Ï J

Uij

(l)

P a r t i c l e i n t e r a c t i o n s are not computed beyond a c u t o f f r a d i u s o f from f o u r t o e i g h t Angstroms t o improve c o m p u t a t i o n a l e f f i c i e n c y by n e g l e c t i n g l o n g - r a n g e i n t e r a c t i o n s which c o n t r i b u t e l i t t l e t o the o v e r a l l s t r u c t u r e of the f l u i d . P e r i o d i c boundary c o n d i t i o n s are imposed by f i l l i n g the space around the b a s i c c e l l w i t h image c e l l s t r a n s l a t e d by m u l t i p l e s o f the u n i t l e n g t h o f the b a s i c c e l l . Thus, p a r t i c l e s near the c e l l b o u n d a r i e s of the b a s i c c e l l i n t e r a c t w i t h image p a r t i c l e s , not w i t h empty c a v i t i e s . This technique prevents s p u r i o u s edge e f f e c t s from a f f e c t i n g the r e s u l t s o f the s i m u l a t i o n . P e r i o d i c c e l l b o u n d a r i e s a l s o a l l o w a s m a l l sample o f p a r t i c l e s to e x h i b i t p r o p e r t i e s c h a r a c t e r i s t i c of a much l a r g e r sample s i z e . A l t h o u g h the method o f moving p a r t i c l e s t o new l o c a t i o n s and o f o b t a i n i n g e q u i l i b r i u m c o n f i g u r a t i o n s f o r the p a r t i c l e s d i f f e r s f o r the MC and MD methods, i n both t e c h n i q u e s the p o s i t i o n s and o r i e n t a t i o n s of thousands o f c o n f i g u r a t i o n s are generated and used t o c a l c u l a t e average p r o p e r t i e s o f the s y s t e m . In the MC method, ensemble average p r o p e r t i e s can be computed. These may i n c l u d e s t r u c t u r a l and thermodynamic p r o p e r t i e s such as d e n s i t y , d i p o l e d i r e c t i o n c o s i n e , hydrogen bond e n e r g i e s and number, r a d i a l d i s t r i b u t i o n f u n c t i o n s , i n t e r n a l e n e r g y , heat c a p a c i t y and i n t e r n a l p r e s s u r e . In the MD method, time average p r o p e r t i e s which are e i t h e r s t r u c t u r a l or dynamic can be computed. For i n s t a n c e , these p r o p e r t i e s may i n c l u d e d e n s i t y , d i p o l e d i r e c t i o n c o s i n e , hydrogen bond e n e r g i e s and number, r a d i a l d i s t r i b u t i o n f u n c t i o n s , d i p o l e r e l a x a t i o n t i m e , and s e l f - d i f f u s i o n coefficients. Thus, the key d i f f e r e n c e between the two t e c h n i q u e s i s t h a t the MC method g e n e r a l l y a l l o w s o n l y s t a t i c p r o p e r t i e s t o be

In Geochemical Processes at Mineral Surfaces; Davis, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

22

G E O C H E M I C A L PROCESSES AT M I N E R A L SURFACES

e v a l u a t e d , w h i l e t h e MD method g e n e r a l l y dependent phenomena to be s t u d i e d .

a l l o w s both s t a t i c and time

Monte C a r l o Methods. A l t h o u g h s e v e r a l s t a t i s t i c a l mechanical ensembles may be s t u d i e d u s i n g MC methods ( 2 , 1 2 , 1 4 ) , t h e c a n o n i c a l ensemble has been the most f r e q u e n t l y used ensemble f o r s t u d i e s o f i n t e r f a c i a l systems. In the c a n o n i c a l ensemble, the number o f m o l e c u l e s ( N ) , c e l l volume (V) and temperature (T) a r e f i x e d . Hence, the c a n o n i c a l ensemble i s denoted by t h e symbols NVT. The c h o i c e o f ensemble determines which thermodynamic p r o p e r t i e s can be computed. In the NVT ensemble one cannot compute the chemical p o t e n t i a l or e n t r o p y o f t h e system; two p r o p e r t i e s which are o f c r i t i c a l importance f o r i n t e r f a c i a l systems. The c h o i c e o f an ensemble a l s o determines the s a m p l i n g a l g o r i t h m used t o generate m o l e c u l a r c o n f i g u r a t i o n s from random moves o f the m o l e c u l e s . In MC methods the u l t i m a t e o b j e c t i v e i s t o e v a l u a t e m a c r o s c o p i c p r o p e r t i e s from i n f o r m a t i o phase s p a c e . To e v a l u a t c a n o n i c a l ensemble from s t a t i s t i c a l m e c h a n i c s , the f o l l o w i n g expression i s used: ρ = Σ

ρ

[ exp(-U (q)/kT)]/Q(N,V,T) Pq

N

(2)

where p i s t h e v a l u e o f the m a c r o s c o p i c p r o p e r t y ρ i n the q t h c o n f i g u r a t i o n , k i s B o l t z m a n n ' s c o n s t a n t , Q(N,V,T) i s the c a n o n i c a l ensemble p a r t i t i o n f u n c t i o n , and t h e summation runs over a l l q e q u i l i b r i u m molecular configurations. Thus, E q u a t i o n 2 suggests t h a t to e v a l u a t e p r o p e r t i e s i n t h e c a n o n i c a l ensemble MC method, t h e p r o b a b i l i t y w i t h which any m o l e c u l a r c o n f i g u r a t i o n o c c u r s s h o u l d be proportional to exp(-U (q)/kT). The s p e c i f i c a l g o r i t h m f o r g e n e r a t i n g new c o n f i g u r a t i o n s t h a t s a t i s f y t h i s requirement i n v o l v e s i ) s e l e c t i n g a m o l e c u l e a t random, i i ) s e l e c t i n g C a r t e s i a n c e n t e r - o f - m a s s d i s p l a c e m e n t c o o r d i n a t e s randomly over an i n t e r v a l which i s not g r e a t e r than h a l f t h e c e l l l e n g t h , i i i ) s e l e c t i n g a r o t a t i o n angle a t random, and i v ) c a l c u l a t i n g t h e new energy, U ( q + l ) , o f t h e new c o n f i g u r a t i o n generated by moves i ) - i i i ) . The f i n a l s t e p i n v o l v e s d e c i d i n g whether t o a c c e p t or r e j e c t the new, but random, configuration. T h i s i s done by g e n e r a t i n g a random number between z e r o and u n i t y and comparing t h e random number t o t h e q u a n t i t y : q

N

N

exp[-(U (q+l) N

- U (q))/kT] N

(3)

I f t h e random number i s l e s s than or equal t o t h e q u a n t i t y i n E q u a t i o n 3, then the new move i s a c c e p t e d ; o t h e r w i s e the move i s r e j e c t e d . T h i s acceptance c r i t e r i a i s o f t e n made even more s t r i n g e n t by r e q u i r i n g t h a t as few as 50% o f the moves s a t i s f y i n g E q u a t i o n 3 a r e actually selected. Note t h a t these procedures always f a v o r moves which l e a d t o reduced t o t a l i n t e r a c t i o n e n e r g i e s . M o l e c u l a r Dynamics Methods. In c o n t r a s t t o the MC method, both k i n e t i c and s t r u c t u r a l p r o p e r t i e s o f a m o l e c u l a r system can be e v a l u a t e d from MD s t u d i e s . These p r o p e r t i e s are e v a l u a t e d as averages over c o n f i g u r a t i o n s generated d u r i n g t i m e . In m i c r o c a n o n i c a l ensemble s t u d i e s w i t h t h e MD method, the p r o p e r t i e s which a r e c o n t r o l l e d

In Geochemical Processes at Mineral Surfaces; Davis, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

2.

MULLA

Simulating

Liquid

Water near Mineral

23

Surfaces

i n c l u d e Ν, V and t o t a l energy, E. T o t a l energy i s computed from the sum of t o t a l k i n e t i c energy and t o t a l p o t e n t i a l energy. The p o t e n t i a l energy i s e v a l u a t e d from an e x p r e s s i o n i n v o l v i n g a summation over the i n t e r a c t i o n p o t e n t i a l s between i n d i v i d u a l p a r t i c l e s i and j , U-jj, as g i v e n i n E q u a t i o n 1. Temperature i s not f i x e d i n the m i c r o c a n o n i c a l MD method, s i n c e i t v a r i e s w i t h f l u c t u a t i o n s i n t o t a l k i n e t i c energy. To determine the movement of m o l e c u l e s , the f o l l o w i n g a l g o r i t h m (15) i s o f t e n u s e d . The f o r c e a c t i n g on the i t h atom i n a m o l e c u l e (F^) i determined from the s p a t i a l d e r i v a t i v e of the t o t a l i n t e r a c t i o n p o t e n t i a l energy of t h a t p a r t i c l e : s

Pi = -V Ej U i j A centered f i n i t e difference p o s i t i o n of the i t h atom Xj(t

+ At)

= -x^t

(4)

scheme i s used to c a l c u l a t e

- At)

+ 2x-j(t) + ( A t / ^ ) ^ 2

the

(5)

where a l l t h a t i s r e q u i r e d to determine t h i s new p o s i t i o n are the p r e s e n t and p a s t l o c a t i o n s , and the f o r c e from E q u a t i o n 4. Typically, to ensure n u m e r i c a l s t a b i l i t y of the a l g o r i t h m , the magnitude of At i s on the o r d e r of 1 0 " seconds or l e s s . The v e l o c i t y of the atom, v-j, i s determined by the e x p r e s s i o n : 1 5

V i

(t)

= [l/UAtOHx^t

+ At)

- Xi(t

- At)]

(6)

Hence, the MD method i n v o l v e s n u m e r i c a l i n t e g r a t i o n of the e q u a t i o n s of motion f o r a l l p a r t i c l e s each time a new c o n f i g u r a t i o n i s g e n e r a t e d , w h i l e the MC method o n l y i n v o l v e s movement of one random p a r t i c l e f o r each new c o n f i g u r a t i o n . I t s h o u l d be noted t h a t many o t h e r n u m e r i c a l a l g o r i t h m s are a v a i l a b l e f o r the MD method ( 1 3 ) , and t h a t maximum a c c u r a c y r e s u l t s from the use of a l g o r i t h m s t h a t i n c l u d e h i g h e r powers of At than are g i v e n i n E q u a t i o n 5. Interaction

Potentials

E q u a t i o n s 3-4 show t h a t the form o f the i n t e r a c t i o n p o t e n t i a l s used i n s i m u l a t i n g i n t e r f a c i a l water i s c r i t i c a l . Of i n t e r e s t f o r i n t e r f a c i a l systems are both the i n t e r a c t i o n p o t e n t i a l between water m o l e c u l e s and t h a t between the s u r f a c e and a water m o l e c u l e . The f i r s t MC (16) and MD (Γ7) s t u d i e s were used to s i m u l a t e the p r o p e r t i e s of s i n g l e p a r t i c l e f l u i d s . A l t h o u g h the b a s i c MC (11,12) and MD (12,13) methods have changed l i t t l e s i n c e the e a r l i e s t s i m u l a t i o n s , the systems s i m u l a t e d have c o n t i n u a l l y i n c r e a s e d i n complexity. The a b i l i t y to s i m u l a t e complex i n t e r f a c i a l systems has r e s u l t e d p a r t l y from improvements i n s i m u l a t i o n a l g o r i t h m s (15,18) or i n the i n t e r a c t i o n p o t e n t i a l s used to model s o l i d s u r f a c e s ( 1 9 ) . The major r e a s o n , however, f o r t h i s a b i l i t y has r e s u l t e d from the i n c r e a s i n g s o p h i s t i c a t i o n of the i n t e r a c t i o n p o t e n t i a l s used to model liquid-liquid interactions. These advances have i n v o l v e d the use of the f o l l o w i n g p o t e n t i a l s : Lennard-Jones 12-6 ( 2 0 ) , Rowlinson ( 2 1 ) , BNS

In Geochemical Processes at Mineral Surfaces; Davis, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

24

G E O C H E M I C A L P R O C E S S E S AT M I N E R A L S U R F A C E S

(22), (28).

ST2 ( 2 3 ) ,

MCY ( 2 4 ) ,

CF ( 2 5 ) ,

PE ( 2 6 ) ,

TIP4P ( 2 7 ) ,

and MCY+CC+DC

Water P o t e n t i a l s . The ST2 ( 2 3 ) , MCY ( 2 4 ) , and CF (25) p o t e n t i a l s are c o m p u t a t i o n a l l y t r a c t a b l e and a c c u r a t e models f o r two-body w a t e r - w a t e r i n t e r a c t i o n p o t e n t i a l s . The ST2, MCY and CF models have f i v e , f o u r , and t h r e e i n t e r a c t i o n s i t e s and have f o u r , t h r e e and t h r e e charge centers, respectively. N e i t h e r the ST2 nor the MCY p o t e n t i a l s a l l o w OH or HH d i s t a n c e s t o v a r y , whereas bond l e n g t h s are f l e x i b l e w i t h the CF model. W h i l e both the ST2 and CF p o t e n t i a l s are e m p i r i c a l models, the MCY p o t e n t i a l i s d e r i v e d from ab i n i t i o c o n f i g u r a t i o n i n t e r a c t i o n m o l e c u l a r o r b i t a l methods (24) u s i n g many g e o m e t r i c a l arrangements of water d i m e r s . The MCY+CC+DC w a t e r - w a t e r p o t e n t i a l (28) i s a r e c e n t m o d i f i c a t i o n of the MCY p o t e n t i a l which a l l o w s f o u r body i n t e r a c t i o n s to be e v a l u a t e d . In comparison to the two-body p o t e n t i a l s d e s c r i b e d above, the MCY+CC+DC p o t e n t i a l r e q u i r e s a supercomputer or a r r a y p r o c e s s o r i n o r d e r t o be c o m p u t a t i o n a l l y f e a s i b l e . T h e r e f o r e , the ST2, MCY and CF p o t e n t i a l the MCY+CC+DC p o t e n t i a l . A comparison of the b u l k water p r o p e r t i e s p r e d i c t e d by the ST2, MCY, and CF models i n s i m u l a t i o n s i s g i v e n i n T a b l e I. These d a t a were o b t a i n e d from ( 2 ) , u n l e s s o t h e r w i s e n o t e d . T a b l e I. Comparison of water p r o p e r t i e s f o r the ST2, MCY and CF s i m u l a t i o n models and b u l k water a t a p p r o x i m a t e l y 298 K. Property

ST2

MCY

CF

b u l k water

-U (kJ/mol) C (J/K/mol) μ (Debye u n i t s ) PV/NkT (V/N=l) D ( 1 0 " m /sec)

34 71 2.35 0.09 3.1 (30)

28.5 79 2.26 8.5 (29) 2.3 (29)

33

34 75 1.86 0.05 2.85

v

9

2

--

1.86 0.1 1.10

(31)

(32)

R e s u l t s i n T a b l e I i l l u s t r a t e some o f the s t r e n g t h s and weaknesses o f the ST2, MCY and CF models. A l l models, except the MCY model, a c c u r a t e l y p r e d i c t the i n t e r n a l e n e r g y , - U . C o n s t a n t volume heat c a p a c i t y , C , i s a c c u r a t e l y p r e d i c t e d by each model f o r which data i s a v a i l a b l e . The ST2 and MCY models o v e r p r e d i c t the d i p o l e moment, μ, w h i l e the CF model p r e d i c t i o n i s i d e n t i c a l w i t h the v a l u e for bulk water. The r a t i o PV/NkT a t a l i q u i d d e n s i t y of u n i t y i s tremendously i n e r r o r f o r the MCY model, w h i l e both the ST2 and CF models p r e d i c t i o n s are r e a s o n a b l e . T h i s l a r g e e r r o r u s i n g the MCY model s u g g e s t s t h a t i t w i l l n o t , i n g e n e r a l , s i m u l a t e thermodynamic p r o p e r t i e s o f water a c c u r a t e l y ( 2 9 ) . V a l u e s o f the s e l f - d i f f u s i o n c o e f f i c i e n t , D, f o r each o f the water models e x c e p t the CF model agree f a i r l y w e l l w i t h the v a l u e f o r b u l k w a t e r . v

In s i m u l a t i n g i n t e r f a c i a l w a t e r , i t i s i m p o r t a n t t o use a model f o r w a t e r - w a t e r i n t e r a c t i o n s which y i e l d s a c c u r a t e r e s u l t s i n s i m u l a t i o n s of bulk water. Each of the models d i s c u s s e d here have o b v i o u s advantages and d i s a d v a n t a g e s . The CF model i s g e n e r a l l y more

In Geochemical Processes at Mineral Surfaces; Davis, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

2.

MULLA

Simulating

Liquid

Water near Mineral

25

Surfaces

a c c u r a t e i n p r e d i c t i n g b u l k water p r o p e r t i e s than the o t h e r models i n Table I. Two drawbacks o f the ST2 model are i t s r i g i d n e s s and o v e r l y t e t r a h e d r a l geometry. The MCY p o t e n t i a l may l e a d to s p u r i o u s r e s u l t s for i n t e r f a c i a l water, since i t generates excessive i n t e r n a l pressures. Surface P o t e n t i a l s . C o n s i d e r the form of the s u r f a c e - w a t e r i n t e r a c t i o n p o t e n t i a l f o r an i n t e r f a c i a l system w i t h a hydrophobic surface. The oxygen atom of any water m o l e c u l e i s a c t e d upon by an e x p l i c i t l y uncharged s u r f a c e d i r e c t l y below i t v i a the Lennard-Jones p o t e n t i a l (U|_j): U (Rij) = A[(a/R L J

i : j

)

a

- (a/R

) ]

(7)

b

i : j

where R^j i s the d i s t a n c e between the j t h s u r f a c e atom and the oxygen atom on the i t h water m o l e c u l e , and A and σ are parameters which s p e c i f y the depth of the p o t e n t i a l energy w e l l and the d i s t a n c e at which i t s v a l u e f i r s t e q u a l b s p e c i f y the power law f o the Lennard-Jones p o t e n t i a l , r e s p e c t i v e l y . Three commonly used p a i r s of v a l u e s f o r a and b are 12 and 6, 9 and 3 , or 4 and 2, which produce the Lennard-Jones 12-6 ( 3 3 , 3 4 ) , 9-3 ( 3 5 ) , and 4-2 (36) p o t e n t i a l s , respectively. T y p i c a l v a l u e s f o r parameters of the l a t t e r L e n n a r d Jones p o t e n t i a l s are r e p o r t e d i n Table I I . In g e n e r a l , the depth of the p o t e n t i a l w e l l f o r these p o t e n t i a l s (about - 0 . 5 kcal/mole) i s t y p i c a l o f the energy between hydrophobic s u r f a c e s and p h y s i s o r b e d noble gases. Table I I . T y p i c a l v a l u e s f o r parameters of the Lennard-Jones 1 2 - 6 , 9 - 3 , and 4-2 p o t e n t i a l s .

Lennard-Jones potential 12-6 9-3 4-2

A (kcal/mole ) 0.303 1.202 1.728

a

b

σ (Angstroms)

12 9 4

6 3 2

3.1 2.5 2.0

The above forms f o r the Lennard-Jones s u r f a c e -water i n t e r a c t i o n p o t e n t i a l have been used as models of hydrophobic s u r f a c e s such as p y r o p h y l l i t e , g r a p h i t e , or p a r a f f i n . I f the i n t e n t i o n o f the s t u d y , however, i s to understand i n t e r f a c i a l p r o c e s s e s a t m i n e r a l s u r f a c e s r e p r e s e n t a t i v e of s m e c t i t e s or m i c a , e x p l i c i t e l e c t r o s t a t i c i n t e r a c t i o n s betweeen water m o l e c u l e s and l o c a l i z e d charges a t the s u r f a c e become i m p o r t a n t . Two methods f o r i n c l u d i n g e x p l i c i t e l e c t r o s t a t i c i n t e r a c t i o n s are proposed. In the f i r s t , and more d i f f i c u l t a p p r o a c h , one would need to conduct e x t e n s i v e quantum mechanical c a l c u l a t i o n s o f the p o t e n t i a l energy v a r i a t i o n between a model s u r f a c e and one a d j a c e n t water molecule u s i n g thousands o f d i f f e r e n t g e o m e t r i c a l o r i e n t a t i o n s . This approach has been used i n a l i m i t e d f a s h i o n to s t u d y the i n t e r a c t i o n p o t e n t i a l between water and s u r f a c e Si-OH groups on a l u m i n o s i 1 i c a t e s , s i l i c a t e s and z e o l i t e s ( 3 7 - 3 9 ) .

In Geochemical Processes at Mineral Surfaces; Davis, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

26

G E O C H E M I C A L PROCESSES AT M I N E R A L SURFACES

A s i m p l e r approach i s t o use an e m p i r i c a l model c o n s i s t i n g o f a Lennard-Jones p o t e n t i a l p l u s a Coulombic term t o e x p l i c i t l y a c c o u n t f o r charges on t h e s u r f a c e . To c a l c u l a t e the magnitude o f t h i s Coulombic c h a r g e , c o n s i d e r t h e p h y s i c a l p r o p e r t i e s o f s m e c t i t e and mica a l u m i n o s i 1 i c a t e s . I t i s known t h a t t h e u n i t c e l l o f s m e c t i t e and mica s u r f a c e s has dimensions o f about 46 square Angstroms ( 3 4 ) . U s i n g t h i s v a l u e f o r t h e a r e a o f a u n i t c e l l and the d a t a i n Table I I I , t h e charge per u n i t c e l l and d e l o c a l i z e d charge on s u r f a c e oxygens o f t y p i c a l s m e c t i t e and mica m i n e r a l s can be computed. Table

Physical

I I I . C a l c u l a t i o n o f t h e number o f charges per u n i t c e l l on t y p i c a l s m e c t i t e and mica s u r f a c e s .

Property

Smectite

s u r f a c e a r e a (m /g) charge d e n s i t y ( e . s . u . / m c a t i o n exchange c a p a c i t y (meq/g charge d e n s i t y (number/unit c e l l ) d e l o c a l i z e d charge per s u r f a c e oxygen 2

Mica

750

100

0.38 0.06

0.96 0.16

To s i m u l a t e s m e c t i t e o r mica m i n e r a l s , a t o t a l o f about 0.4 and 1 e x p l i c i t n e g a t i v e c h a r g e s , r e s p e c t i v e l y , need t o be a s s i g n e d t o each u n i t c e l l on t h e s u r f a c e . T h i s charge s h o u l d be d e l o c a l i z e d over about s i x oxygen atoms s u r r o u n d i n g t h e d i t r i g o n a l c a v i t i e s o f t h e s m e c t i t e and mica s u r f a c e s , s i n c e these charges o r i g i n a t e from o c t a h e d r a l o r t e t r a h e d r a l s i t e s w i t h i n t h e c r y s t a l and n o t from t h e s u r f a c e atoms. A proposed form f o r t h e w a t e r - s u r f a c e i n t e r a c t i o n p o t e n t i a l , U ^ , s u i t a b l e f o r s i m u l a t i o n s o f s m e c t i t e o r mica s u r f a c e s i n t e r a c t i n g w i t h t h e ST2 model o f water i s : 4 U

WS( ijAj) R

= A[(a/R

i ; j

)

a

-

(o/R

i : j

) ] + S(RT j ) b

Σ

(q qj) / d a

a j

(8)

a=l where R ^ j , A , a , b and σ a r e as d e f i n e d i n E q u a t i o n 7 and T a b l e I I , d j i s t n e d i s t a n c e between t h e j t h s u r f a c e atom and t h e a t h charge ( h a v i n g charge q ) on t h e water m o l e c u l e , qj i s t h e d e l o c a l i z e d charge on t h e j t h s u r f a c e atom from T a b l e I I I , and S(R^j ) i s t h e s w i t c h i n g f u n c t i o n o f t h e ST2 water p o t e n t i a l ( 2 3 ) . The magnitude o f charge on each o f t h e f o u r p o i n t charges f o r ST2 water i n E q u a t i o n 8 i s 0.2357. a

a

A p l o t o f t h e Lennard-Jones 9-3 form o f E q u a t i o n s 7 and 8 f o r ST2 water i n t e r a c t i n g w i t h s m e c t i t e and mica s u r f a c e s i s shown i n F i g u r e 1. V a l u e s f o r the parameters used i n F i g u r e 1 a r e g i v e n i n T a b l e s II and I I I , and i n r e f e r e n c e ( 2 3 ) . The water m o l e c u l e i s o r i e n t e d so t h a t i t s p r o t o n s f a c e t h e s u r f a c e and i t s lone p a i r e l e c t r o n s f a c e away from t h e s u r f a c e , and t h e p r o t o n s a r e e q u i d i s t a n t from t h e s u r f a c e . Note t h a t t h e depth o f t h e p o t e n t i a l w e l l i n F i g u r e 1 f o r i n t e r a c t i o n s w i t h t h e s m e c t i t e s u r f a c e and mica s u r f a c e a r e

In Geochemical Processes at Mineral Surfaces; Davis, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

MULLA

Simulating

-4

Liquid

ι 0

.

Water near Mineral

ι

2

.

ι

4

.

ι

6

Surfaces

.

ι

8

,

I

10

DISTRNCE (Angstroms) F i g u r e 1. Comparison o f ST2 w a t e r - s u r f a c e i n t e r a c t i o n s computed from E q u a t i o n s 7 o r 8 u s i n g parameters f o r the Lennard-Jones 9-3 p o t e n t i a l i n Table II and the d e l o c a l i z e d charge magnitude f o r s m e c t i t e and mica s u r f a c e s i n Table III.

In Geochemical Processes at Mineral Surfaces; Davis, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

28

G E O C H E M I C A L PROCESSES AT M I N E R A L SURFACES

about - 1 . 5 and - 3 . 5 k c a l / m o l e , r e s p e c t i v e l y . These i n t e r a c t i o n e n e r g i e s a r e s i m i l a r i n magnitude t o weak hydrogen bond e n e r g i e s . An i n t e r a c t i o n p o t e n t i a l between t h e s u r f a c e and i o n s may a l s o be needed i n s i m u l a t i n g c o u n t e r i o n d i f f u s i o n f o r t h e s m e c t i t e and mica s u r f a c e models. The form o f such an i n t e r a c t i o n p o t e n t i a l remains t o be d e t e r m i n e d . T h i s may n o t pose a s i g n i f i c a n t p r o b l e m , s i n c e r e c e n t e v i d e n c e (40) suggests t h a t over 98% o f t h e c a t i o n s near s m e c t i t e s u r f a c e s l i e w i t h i n t h e shear p l a n e . For s p e c i f i c a l l y adsorbed c a t i o n s such as p o t a s s i u m or c a l c i u m , t h e s u r f a c e - i o n i n t e r a c t i o n s can a l s o be n e g l e c t e d i f i t i s assumed t h a t c a t i o n d i f f u s i o n c o n t r i b u t e s l i t t l e t o t h e water s t r u c t u r e . In s i m u l a t i n g t h e i n t e r a c t i o n p o t e n t i a l between c o u n t e r i o n s and i n t e r f a c i a l w a t e r , a w a t e r - i o n i n t e r a c t i o n p o t e n t i a l s i m i l a r t o those a l r e a d y developed f o r MD s i m u l a t i o n s (41-43) c o u l d be s p e c i f i e d . Simulations of I n t e r f a c i a l

Water

S e v e r a l MC and MD s t u d i e s u r f a c e s have been r e p o r t e d ( 3 3 - 3 6 , 4 4 - 4 8 ) . Both o f t h e MC s t u d i e s ( 3 5 , 4 5 ) , as w e l l as the f o u r MD s t u d i e s ( 3 3 , 3 4 , 3 6 , 4 7 ) r e p o r t i n g d e t a i l e d o b s e r v a t i o n s o f i n t e r f a c i a l water are d i s c u s s e d h e r e . This comparison w i l l show t h a t c h o i c e o f t h e w a t e r - w a t e r p o t e n t i a l i s c r i t i c a l f o r such s t u d i e s . I t w i l l a l s o i l l u s t r a t e t h e wide range o f i n t e r f a c i a l p r o p e r t i e s which can be s t u d i e d u s i n g computer simulations. R e s u l t s from t h e e a r l y p i o n e e r i n g MC s t u d i e s f o r i n t e r f a c i a l water a r e summarized i n T a b l e I V . Table

IV.

R e s u l t s from Monte C a r l o S i m u l a t i o n s o f Water Hydrophobic S u r f a c e s .

# molecules c e l l dimensions (nm^) c e l l d e n s i t y (g/cc) temperature (K) # configurations water p o t e n t i a l surface potential range o f d e n s i t y o s c i l l a t i o n s (g/cc) d e n s i t y t r e n d towards s u r f a c e s hydrogen bonding t r e n d towards s u r f a c e s i n t e r n a l energy t r e n d towards s u r f a c e s preferred dipole orientation near s u r f a c e s

Near

Reference 35

Reference 45

216 7.127 0.906 298 2.5 χ 1 0 Row!inson L - J 9-3 1.1 t o 1.5 decreases decreases decreases

150 4.5 0.997 300 1 χ 10 MCY hard w a l l 0.5 t o 2 . 3 increases increases increases

yes

yes

6

4

These r e s u l t s i n d i c a t e t h a t , compared t o b u l k w a t e r , i n t e r f a c i a l water e x h i b i t s unique o s c i l l a t i o n s i n d e n s i t y w i t h d i s t a n c e from t h e s u r f a c e and p r e f e r e n t i a l d i p o l a r o r i e n t a t i o n s . Both s i m u l a t i o n s r e p o r t d e n s i t y v a l u e s which a r e u n r e a s o n a b l e . P a r t o f t h i s problem a r i s e s from a t t e m p t i n g t o f i x t h e water d e n s i t y based on t h e average c e l l volume and t h e number o f water m o l e c u l e s ; an approach which

In Geochemical Processes at Mineral Surfaces; Davis, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

2.

MULLA

Simulating

Liquid

Water near Mineral

29

Surfaces

o v e r l o o k s the f a c t t h a t the c e l l volume near the s u r f a c e s i s g e n e r a l l y f r e e of water m o l e c u l e s due to r e p u l s i v e f o r c e s a r i s i n g from the surfaces. For the study u s i n g the MCY p o t e n t i a l ( 4 5 ) , a more s e r i o u s problem i n v o l v e s the e x c e s s i v e i n t e r n a l p r e s s u r e s generated by the MCY p o t e n t i a l , which l e a d to e x c e s s i v e d e n s i t y o s c i l l a t i o n s and i n c r e a s i n g d e n s i t y near the s u r f a c e s . C o n s i s t e n c y between the two s i m u l a t i o n s i s poor; whereas the f i r s t (35) p r e d i c t s decreased d e n s i t y , hydrogen bonding and i n t e r n a l energy near the s u r f a c e s , the second (45) r e p o r t s e x a c t l y the o p p o s i t e t r e n d s . These d i f f e r e n c e s are a l l p r o b a b l y r e l a t e d to the use of d i f f e r e n t w a t e r - w a t e r p o t e n t i a l s . R e s u l t s o f s e l e c t e d MD s t u d i e s o f i n Table V. Table V.

i n t e r f a c i a l water are

R e s u l t s from S e l e c t e d M o l e c u l a r Dynamics S t u d i e s o f Near Hydrophobic S u r f a c e s .

(47) # molecules c e l l volume (ntrr*) c e l l d e n s i t y (g/cc) temperature (K) t r a j e c t o r y time (ps) water p o t e n t i a l surface p o t e n t i a l range of d e n s i t y o s c i l l a t i o n s (g/cc) density trend towards s u r f a c e s hydrogen bonding t r e n d towards s u r f a c e s preferred dipolar orientations self-diffusion coeff. near s u r f a c e s (m /s) near midplane (m /s) d i p o l e r e l a x a t i o n time near s u r f a c e s (ps) near midplane (ps) 2

2

(33)

(34) 256 9.156 0.84 286 0.75 ST2 L - J 12-6

216 8.787 0.74 287 20 ST2 L - J 12-6

150 5.198 0.87 304 14 MCY L - J 4-2

0.9 to

0.9 t o

0.5 to

1.0

Water

(36)

150 4.5 1.0 301 25 ST2 hard w a l l 1.0

reported

3.2

0.8 to

1.1

decreases

decreases

increases

decreases

—

—

—

decreases

yes

yes

yes

yes

3.3 χ 1 0 " 4.2 χ 1 0 " 3.1 2.1

9 9

4.8 χ 1 0 " 3.3 χ 1 0 " — —

9 9

3.1 χ 1 0 " 3.7 χ 1 0 "

9 9

2.3 2.0

2.1 χ 1 0 " 2.7 χ 1 0 " — —

The r e s u l t s i n Table V i l l u s t r a t e t h a t MD s t u d i e s , compared t o the MC r e s u l t s i n Table IV, f a c i l i t a t e the i n v e s t i g a t i o n o f t r a n s p o r t and time-dependent p r o p e r t i e s . A l s o , they show t h a t use of the MCY p o t e n t i a l l e a d s to v e r y l a r g e d e n s i t y o s c i l l a t i o n s and i n c r e a s i n g water d e n s i t y near the s u r f a c e s . T h i s appears t o be a s e r i o u s drawback t o the use of the MCY p o t e n t i a l i n s i m u l a t i o n s of i n t e r f a c i a l water. R e s u l t s from the i n v e s t i g a t i o n s u s i n g the ST2 p o t e n t i a l show t h a t i n t e r f a c i a l water d e n s i t y i s a p p r o x i m a t e l y 1.0 g/cc, w i t h a tendency f o r decreased d e n s i t y and hydrogen bonding near the s u r f a c e s . As i n the MC s i m u l a t i o n s , o r i e n t a t i o n s o f the water d i p o l e moment are a f f e c t e d by the presence of a s o l i d / l i q u i d i n t e r f a c e , and an

In Geochemical Processes at Mineral Surfaces; Davis, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

9 9

30

G E O C H E M I C A L P R O C E S S E S AT M I N E R A L S U R F A C E S

a p p r e c i a b l e decrease i n d i p o l e r e l a x a t i o n and the water c o e f f i c i e n t are u s u a l l y observed near the s u r f a c e s .

self-diffusion

Comparison w i t h E x p e r i m e n t . How do the r e s u l t s of these s i m u l a t i o n s compare w i t h e x p e r i m e n t a l r e s u l t s on c o r r e s p o n d i n g s y s t e m s , and what do they i n f e r about i n t e r p r e t i n g the " h y d r o p h o b i c e f f e c t " , " h y d r a t i o n f o r c e s " , and the " s t r u c t u r e " of i n t e r f a c i a l water? S t r u c t u r a l l y , the i n t e r f a c i a l water e x h i b i t s c l e a r d e n s i t y o s c i l l a t i o n s which extend t o a t l e a s t 15 Angstroms from the s u r f a c e s ( 3 4 ) . Since t h i s s i m u l a t i o n i n v o l v e d a h y d r o p h o b i c , n e u t r a l s u r f a c e , these e f f e c t s are d i r e c t l y a t t r i b u t a b l e t o the presence of the s u r f a c e s , and not to an e f f e c t o f charged c o u n t e r i o n s . F u r t h e r m o r e , s i n c e no l o n g - r a n g e changes i n hydrogen bonding p a t t e r n s were observed due to t h i s s t r u c t u r a l r e o r d e r i n g ( 3 4 ) , the MD r e s u l t s suggest t h a t the hydrophobic e f f e c t i s due t o e n t r o p y changes i n the i n t e r f a c i a l l i q u i d r a t h e r than to l o n g range bonding e f f e c t s . The presence of d e n s i t i n t e r f a c i a l water has s t i m u l a t e e x p l i c i t l y designed to d e t e c t t h e i r p r e s e n c e . Structural forces a s s o c i a t e d w i t h d e n s i t y o s c i l l a t i o n s i n water next t o mica s u r f a c e s have r e c e n t l y been measured e x p e r i m e n t a l l y ( 4 9 ) . A l t h o u g h , mica i s not a hydrophobic s u r f a c e , i t s h o u l d be p o i n t e d out t h a t t h e r e i s t h e o r e t i c a l b a s i s f o r s u g g e s t i n g t h a t m o l e c u l a r l a y e r i n g near s u r f a c e s and the accompanying o s c i l l a t o r y f o r c e s are r e s p o n s i b l e f o r both the " h y d r a t i o n " and " h y d r o p h o b i c " e f f e c t s ( 2 ) . Whether t h i s f o r c e i s a t t r a c t i v e (hydrophobic e f f e c t ) or r e p u l s i v e ( h y d r a t i o n e f f e c t ) depends upon how the d e n s i t y o s c i l l a t i o n s f i t i n t o the r e g i o n between the s u r f a c e s . Much MC work i s needed w i t h both hydrophobic and h y d r o p h i l i c s u r f a c e s u s i n g the grand c a n o n i c a l ensemble to determine the chemical p o t e n t i a l and e n t r o p y of the i n t e r f a c i a l water a t v a r i o u s s u r f a c e s e p a r a t i o n s i n o r d e r t o b e t t e r understand the magnitude of these e f f e c t s . T h e o r e t i c a l e x p l a n a t i o n s f o r the " h y d r a t i o n f o r c e " (50) and the " h y d r o p h o b i c e f f e c t " (51) o f t e n i n v o l v e an a n a l y s i s of the f o r c e s emanating from o r i e n t e d m o l e c u l a r d i p o l e or quadrupole moments. All o f the computer s i m u l a t i o n s f o r i n t e r f a c i a l water d i s c u s s e d above found s i g n i f i c a n t e v i d e n c e f o r p r e f e r r e d d i p o l a r o r i e n t a t i o n s near the surfaces. In most o f the s t u d i e s , a l l o f which used n o n - p o l a r i z a b l e , p a i r w i s e i n t e r a c t i n g water models, the tendency f o r p r e f e r r e d d i p o l a r o r i e n t a t i o n s diminishes in a continuous fashion with i n c r e a s i n g d i s t a n c e from the s u r f a c e s and was n e g l i g i b l e a t a d i s t a n c e of from ten to f i f t e e n Angstroms from the s u r f a c e s . When r e a l i s t i c water p o t e n t i a l s i n c o r p o r a t i n g c o o p e r a t i v e e f f e c t s become a v a i l a b l e , t h i s e f f e c t can be expected to become even more s i g n i f i c a n t . A c c o r d i n g t o the Kirkwood t h e o r y of p o l a r d i e l e c t r i c s , s i m p l e r e l a t i o n s (23) between m o l e c u l a r d i p o l e moment v e c t o r s and the meansquare t o t a l d i p o l e moment o f water c l u s t e r s can be used to compute the s t a t i c d i e l e c t r i c c o n s t a n t of w a t e r . As the n o r m a l i z e d meansquare t o t a l d i p o l e moment i n c r e a s e s towards u n i t y , t h e o r y p r e d i c t s d e c r e a s e s i n the s t a t i c d i e l e c t r i c c o n s t a n t . S i n c e MD r e s u l t s i n d i c a t e t h a t the mean-square t o t a l d i p o l e moment of i n t e r f a c i a l water i s g r e a t e r than t h a t f o r b u l k water ( 4 8 ) , the s t a t i c d i e l e c t r i c

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c o n s t a n t of i n t e r f a c i a l water s h o u l d be lower than t h a t i n b u l k w a t e r . T h i s c o n c l u s i o n i s meant to be o n l y q u a l i t a t i v e , s i n c e c a l c u l a t i o n s of the s t a t i c d i e l e c t r i c c o n s t a n t u s i n g the Kirkwood t h e o r y may be i n c o n s i d e r a b l e e r r o r i f the e f f e c t s of the r e a c t i o n f i e l d are not accounted f o r ( 5 2 ) . The r e a c t i o n f i e l d i s an e l e c t r i c a l f i e l d w i t h i n the c u t o f f r a d i u s which c o n t r i b u t e s t o the d i e l e c t r i c c o n s t a n t . It r e s u l t s from the p o l a r i z i n g e f f e c t s o f the d i p o l e moments o u t s i d e the c u t o f f r a d i u s , and i t s e f f e c t can be i n c l u d e d ( w i t h c o n s i d e r a b l e e f f o r t ) i f a c c u r a t e computations of the d i e l e c t r i c c o n s t a n t are desired (53). The heterogeneous d i e l e c t r i c p r o p e r t i e s i n the l i q u i d medium near s u r f a c e s have i m p o r t a n t i m p l i c a t i o n s f o r t h e o r i e s of the s t r u c t u r e of the i n t e r f a c i a l r e g i o n . A r e c e n t t h e o r e t i c a l s t u d y of the e f f e c t of decreased medium d i e l e c t r i c c o n s t a n t near s u r f a c e s (54) shows t h a t i t i s associated with s i g n i f i c a n t reductions in surface potential computed from double l a y e r t h e o r y . A note of c a u t i o n to t h e o r e t i c i a n s is in order. The m o l e c u l a d i e l e c t r i c p r o p e r t i e s of i n c r e a s i n g d i s t a n c e from the s u r f a c e . Hence, the use o f d i s c r e t e m i x t u r e models (54,55) i n which water near the s u r f a c e s i s d i v i d e d i n t o two z o n e s ; one having p r o p e r t i e s c h a r a c t e r i s t i c of water i n the f i r s t adsorbed l a y e r and the second h a v i n g b u l k p r o p e r t i e s are not l i k e l y to represent actual s u r f a c i a l c o n d i t i o n s . Similar caution s h o u l d be used i n a d s o r p t i o n models t h a t assume d i s c r e t e m o l e c u l a r l a y e r s of s u r f a c e complexes i n the i n t e r f a c i a l r e g i o n . R e a c t i o n k i n e t i c s and many t r a n s p o r t p r o p e r t i e s i n l i q u i d s are c o n t r o l l e d by r a t e s of d i f f u s i o n . P r o c e s s e s t h a t may be c o n t r o l l e d by d i f f u s i o n i n c l u d e , f o r example, r a t e s of l i g a n d exchange from t r a n s i t i o n metal i o n s ( 5 6 ) , r e a c t i o n s i n v o l v i n g p r o t o n t r a n s f e r ( 5 7 ) , and exchange r e a c t i o n s near m i n e r a l s u r f a c e s ( 5 8 ) . R e s u l t s i n Table V (from s t u d i e s 3 4 , 3 6 , 4 7 ) i n d i c a t e t h a t the v a l u e f o r the s e l f - d i f f u s i o n c o e f f i c i e n t of water near m i n e r a l s u r f a c e s i s c o n s i s t e n t l y about 80% lower than the v a l u e near the m i d p l a n e . Q u a n t i t a t i v e l y , the v a l u e s f o r D near the midplane are h i g h e r than v a l u e s r e p o r t e d f o r b u l k water at comparable temperatures ( 5 5 ) . For i n s t a n c e , the v a l u e s o f D i n b u l k water a t 285 and 300 Κ are about 1.8 and 2.9 χ 1 0 ~ m / s , respectively. The r e s u l t s from one study ( 3 3 ) , appear to be e x c e s s i v e l y h i g h , c o n s i d e r i n g the temperature of the s i m u l a t i o n , and are a l s o i n c o n s i s t e n t w i t h e x p e r i m e n t a l r e s u l t s i n t h a t they p r e d i c t d i f f u s i o n r a t e s which are g r e a t e r near the s u r f a c e s than near the midplane. E x c l u d i n g the l a t t e r r e s u l t s , v a l u e s f o r D from the MD s i m u l a t i o n s appear to q u a l i t a t i v e l y obey the expected t r e n d f o r i n c r e a s i n g v a l u e s of D w i t h i n c r e a s i n g t e m p e r a t u r e . F u r t h e r m o r e , the decreases i n v a l u e s f o r s e l f - d i f f u s i o n c o e f f i c i e n t near the s u r f a c e s are q u a l i t a t i v e l y c o n s i s t e n t w i t h e x p e r i m e n t a l measurements of decreased water m o b i l i t y near n e u t r a l s i l i c a t e s u r f a c e s ( 5 9 - 6 2 ) . 9

2

Another t r a n s p o r t p r o p e r t y of i n t e r f a c i a l water which can be s t u d i e d by MD t e c h n i q u e s i s the d i p o l e r e l a x a t i o n t i m e . This property i s computed from the d i p o l e moment c o r r e l a t i o n f u n c t i o n , which measures the r a t e a t which d i p o l e moment a u t o c o r r e l a t i o n i s l o s t due to r o t a t i o n a l motions i n time ( 6 3 ) . L a r g e r v a l u e s f o r the d i p o l e r e l a x a t i o n time i n d i c a t e slower r o t a t i o n a l motions o f the d i p o l e

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G E O C H E M I C A L P R O C E S S E S AT M I N E R A L S U R F A C E S

moment. Both of the MD s t u d i e s r e p o r t i n g v a l u e s f o r the d i p o l e r e l a x a t i o n time i n Table V i n d i c a t e t h a t r e l a x a t i o n times are l a r g e r f o r water near the s u r f a c e s than f o r water near the m i d p l a n e . The MCY p o t e n t i a l , however, i s not as s e n s i t i v e t o changes i n r e l a x a t i o n time as i s the ST2 p o t e n t i a l . These r e s u l t s can be i n t e r p r e t e d to mean t h a t m o l e c u l e s near the s u r f a c e s e x p e r i e n c e h i n d e r e d r o t a t i o n a l movement as compared t o water near the m i d p l a n e . Experimental e v i d e n c e from near i n f r a r e d ( 6 ) , e l e c t r o n s p i n resonance ( 5 9 ) , and n u c l e a r magnetic resonance (60) s t u d i e s s u p p o r t the MD s i m u l a t i o n r e s u l t s , i n t h a t they i n d i c a t e h i n d e r e d r o t a t i o n a l motion of water near uncharged s i l i c a t e s u r f a c e s . Summary Monte C a r l o and M o l e c u l a r Dynamics s i m u l a t i o n s of water near hydrophobic s u r f a c e s have y i e l d e d a w e a l t h of i n f o r m a t i o n about the s t r u c t u r e , thermodynamics and t r a n s p o r t p r o p e r t i e s of i n t e r f a c i a l water. In p a r t i c u l a r , the m o l e c u l a r l a y e r i n g and d e n s i t Angstroms away from the s u r f a c e s . These o s c i l l a t i o n s have r e c e n t l y been v e r i f i e d e x p e r i m e n t a l l y . Ordered d i p o l a r o r i e n t a t i o n s and reduced d i p o l e r e l a x a t i o n times are observed i n most of the s i m u l a t i o n s , i n d i c a t i n g t h a t i n t e r f a c i a l water i s not a u n i f o r m d i e l e c t r i c continuum. Reduced d i p o l e r e l a x a t i o n times near the s u r f a c e s i n d i c a t e t h a t i n t e r f a c i a l water e x p e r i e n c e s h i n d e r e d rotation. The m a j o r i t y of s i m u l a t i o n r e s u l t s i n d i c a t e t h a t water near hydrophobic s u r f a c e s e x h i b i t s fewer hydrogen bonds than water near the midplane. S e v e r a l m e r i t s and s t r e n g t h s of m o l e c u l a r s i m u l a t i o n s of i n t e r f a c i a l water are a p p a r e n t . S i n c e these methods y i e l d s t r u c t u r a l , thermodynamic and t r a n s p o r t p r o p e r t i e s f o r b u l k water which are i n good agreement w i t h many e x p e r i m e n t a l l y measured p r o p e r t i e s of b u l k water over a wide temperature range, they seem t o o f f e r a p r o m i s i n g approach f o r s t u d y i n g i n t e r f a c i a l w a t e r . I n t e r f a c i a l systems are g e n e r a l l y composed of s e v e r a l components and are d i f f i c u l t t o characterize. The n a t u r e of m o l e c u l a r s i m u l a t i o n s a l l o w s the system b e i n g a n a l y z e d to be e x a c t l y s p e c i f i e d i n terms of the types o f components, t h e i r i n t e r a c t i o n p o t e n t i a l s , the i n i t i a l atomic or m o l e c u l a r l o c a t i o n s and the types o f boundary c o n d i t i o n s imposed. Thus, e f f e c t s o f the s u r f a c e s can be s t u d i e d i n d e t a i l , s e p a r a t e l y from e f f e c t s o f c o u n t e r i o n s or s o l u t e s . In a d d i t i o n , i n d i v i d u a l l a y e r s o f i n t e r f a c i a l water can be a n a l y z e d as a f u n c t i o n of d i s t a n c e from the s u r f a c e and d i r e c t i o n a l a n i s o t r o p y i n v a r i o u s p r o p e r t i e s can be s t u d i e d . F i n a l l y , one computer experiment can o f t e n y i e l d i n f o r m a t i o n on s e v e r a l water p r o p e r t i e s , some o f which would be t i m e consuming or even i m p o s s i b l e t o o b t a i n by e x p e r i m e n t a t i o n . Examples of i n t e r f a c i a l water p r o p e r t i e s which can be computed v i a the MD s i m u l a t i o n s but not v i a experiment i n c l u d e the number of hydrogen bonds per m o l e c u l e , v e l o c i t y a u t o c o r r e l a t i o n f u n c t i o n s , and r a d i a l distribution functions. S e v e r a l weaknesses and d i s a d v a n t a g e s o f the computer s i m u l a t i o n methods can a l s o be mentioned. Foremost among these l i m i t a t i o n s i s the f a c t t h a t none o f the commonly used models f o r water i n t e r a c t i o n s

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account f o r t h r e e - or f o u r - b o d y i n t e r a c t i o n s . Y e t , the complex b e h a v i o u r of i n t e r f a c i a l water i s a p p a r e n t l y d i c t a t e d by these cooperative e f f e c t s (26). S i g n i f i c a n t d i f f e r e n c e s e x i s t i n the i n t e r n a l energy, hydrogen bonding and d i f f u s i o n r a t e s o f i n t e r f a c i a l water as p r e d i c t e d by the ST2 and MCY p o t e n t i a l s , w i t h r e s u l t s f o r the former model being more c o n s i s t e n t w i t h e x p e r i m e n t a l r e s u l t s than those f o r the l a t t e r . These d i f f e r e n c e s p r o b a b l y r e l a t e to excess i n t e r n a l p r e s s u r e s generated by the MCY p o t e n t i a l . A second problem i n v o l v e s u n c e r t a i n t y about what d i s t a n c e o f s e p a r a t i o n s h o u l d be imposed between s u r f a c e s f o r a p a r t i c u l a r c h o i c e o f the number o f water m o l e c u l e s . I f e x c l u d e d volume near the s u r f a c e s i s not accounted f o r , the r e s u l t i n g average water d e n s i t y and i t s o s c i l l a t i o n s w i l l be e x c e s s i v e . F i n a l l y , d i e l e c t r i c p r o p e r t i e s of i n t e r f a c i a l water are not e a s i l y q u a n t i f i e d when r i g i d , p a i r w i s e i n t e r a c t i n g water p o t e n t i a l s are u s e d . T h i s l i m i t a t i o n can be p a r t i a l l y overcome by a c c o u n t i n g f o r r e a c t i o n f i e l d e f f e c t s , but even t h e n , water i n t e r a c t i o n p o t e n t i a l s i n c u r r e n t use are not e f f e c t i v e i n modeling the d i e l e c t r i c p r o p e r t i e On the w h o l e , the advantages and s t r e n g t h s of MC and MD s i m u l a t i o n s of i n t e r f a c i a l water outweigh t h e i r d i s a d v a n t a g e s and weaknesses. Even i f q u a n t i t a t i v e p r e d i c t i o n o f i n t e r f a c i a l water p r o p e r t i e s i s not p o s s i b l e i n some c a s e s , a knowledge of q u a l i t a t i v e t r e n d s as a f u n c t i o n o f d i s t a n c e from the s u r f a c e s or r e l a t i v e t o r e s u l t s from s i m u l a t i o n s o f b u l k water are o f t e n e x t r e m e l y illuminating. What i s the l i k e l y f u t u r e use o f MC and MD t e c h n i q u e s f o r s t u d y i n g i n t e r f a c i a l systems? S e v e r a l p r o m i s i n g approaches are possible. C o n t i n u e d i n v e s t i g a t i o n o f double l a y e r p r o p e r t i e s , " h y d r a t i o n f o r c e s " , " h y d r o p h o b i c e f f e c t s " , and " s t r u c t u r e d w a t e r " are c l e a r l y a w a i t i n g the development of improved models f o r w a t e r - w a t e r , s o l u t e - w a t e r , s u r f a c e - w a t e r , and s u r f a c e - s o l u t e p o t e n t i a l s . S i m u l a t i o n s o f o r g a n i c s near s u r f a c e s are c l e a r l y p o s s i b l e s i n c e p o t e n t i a l s d e s c r i b i n g w a t e r - o r g a n i c i n t e r a c t i o n s are p r e s e n t l y available (64,65). S t u d i e s o f r e a c t i o n k i n e t i c s and l i g a n d exchange p r o c e s s e s near s u r f a c e s are c l e a r l y p o s s i b l e u s i n g the m o l e c u l a r t i m e s c a l e g e n e r a l i z e d Langevin e q u a t i o n approach ( 6 6 ) . Gaseous a d s o r p t i o n on metal ( 6 7 ) , hydrophobic (68) and o t h e r s i m p l e s u r f a c e s have been e x t e n s i v e l y s t u d i e d ( 2 ) , but s i m i l a r approaches u s i n g models f o r c l a y and z e o l i t e s u r f a c e s are a l s o p o s s i b l e . Mechanisms o f c r y s t a l growth and d e f e c t s t r u c t u r e s i n v i t r e o u s s i l i c a (69) and g l a s s (70) have been s t u d i e d , and s i m i l a r s t u d i e s on a l u m i n o s i l i c a t e m i n e r a l s (even under c o n d i t i o n s of h i g h temperature and p r e s s u r e ) are p o s s i b l e . Finally, new t h e o r e t i c a l developments are a l l o w i n g thermodynamic p r o p e r t i e s (71,72) and n o n - e q u i l i b r i u m c o n d i t i o n s (73) to be s t u d i e d w i t h MD methods. In s h o r t , MC and MD s t u d i e s of i n t e r f a c i a l systems are s t i l l in t h e i r infancy.

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L i s t o f Symbols a A b C d j D Ε v

a

k m-j Ν Pq ρ Ρ qj q Q R-jj S(R.jj) Τ U U-jj U|_j U|\j U 5 v.j V x-j μ σ At a

W

parameter f o r t h e Lennard-Jones p o t e n t i a l parameter f o r t h e Lennard-Jones p o t e n t i a l parameter f o r t h e Lennard-Jones p o t e n t i a l c o n s t a n t volume heat c a p a c i t y d i s t a n c e between s u r f a c e and a t h charge on water m o l e c u l e self-diffusion coefficient t o t a l energy f o r c e a c t i n g on i t h atom Boltzmann's constant mass o f i t h atom number o f m o l e c u l e s value of pth property of qth configuration ensemble average v a l u e o f p r o p e r t y ρ i n t e r n a l pressure charge on j t h s u r f a c e atom charge on a t h c a n o n i c a l ensembl d i s t a n c e between i t h and j t h atoms s w i t c h i n g f u n c t i o n o f ST2 water p o t e n t i a l a b s o l u t e temperature i n t e r n a l energy pairwise interaction potential Lennard-Jones i n t e r a c t i o n p o t e n t i a l t o t a l i n t e r a c t i o n p o t e n t i a l energy p o t e n t i a l energy f o r water i n t e r a c t i n g w i t h a charged s u r f a c e v e l o c i t y o f i t h atom volume p o s i t i o n o f i t h atom d i p o l e moment parameter f o r t h e Lennard-Jones p o t e n t i a l time s t e p f o r MD a l g o r i t h m

Literature Cited 1. 2.

Cairns-Smith, A. G. Scientific American 1985; 252(6), 90-100. Nicholson, D.; Parsonage, N. G. "Computer Simulation and the Statistical Mechanics of Adsorption"; Academic: New York, 1982; Chap. 4,6. 3. van Megen, W.; Snook, I. Adv. Colloid Interface Sci. 1984; 21, 119-194. 4. Low, P. F. Adv. Agronomy 1961, 13, 269-327. 5. Graham, J. Rev. Pure Appl. Chem. 1964; 14, 81-89. 6. Klier, K.; Zettlemoyer, A. C. J. Colloid Interface Sci. 1977; 58(2), 216-229. 7. Low, P. F. Soil Sci. Soc. Am. J. 1979; 43(5), 651-658. 8. Derjaguin, Β. V.; Churaev, Ν. V. In "Progress in Surface and Membrane Science"; Cadenhead, D. Α.; Danielli, J. F., Eds.; Academic: New York, 1981; Vol. 14, pp 69-130. 9. Sposito, G.; Prost, R. Chem. Rev. 1982; 82(6), 553-573. 10. Void, R. D.; Void, M. J. "Colloid and Interface Chemistry"; Addison-Wesley: Reading, Mass., 1983; Chap. 7,9.

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Valleau, J. P.; Whittington, S. G. In "Modern Theoretical Chemistry"; Berne, B. J., Ed.; Plenum: New York, 1977; Vol. 5, Chap. 4. Barker, J. A.; Henderson, D. Rev. Mod. Phys. 1976; 48(4), 587671. Wood, D. W. In "Water: A Comprehensive Treatise"; Franks, F., Ed.; Plenum: New York, 1979; Vol. 6, Chap. 6. Snook, I. K.; van Megen, W. J. Chem. Phys. 1980; 72(5), 2907-2913. Verlet, L. Phys. Rev. 1967; 159(1), 98-103. Metropolis, N.; Rosenbluth, A. W.; Rosenbluth, M. N.; Teller, A. H. J. Chem. Phys. 1953; 21(6), 1087-1092. Alder, B. J.; Wainwright, T. E. J. Chem. Phys. 1957; 27, 12071209. Ryckaert, J. P.; Ciccotti, G.; Berendsen, H. J. C. J. Comp. Phys. 1977; 23, 327-341. Steele, W. A. Surface Sci. 1973; 36, 317-352. Rahman, A. Phys. Rev. 1964; 136(2A), A405-A411. Barker, J. Α.; Watts 145. Rahman, Α.; Stillinger, F. H. J. Chem. Phys. 1971; 55(7), 33363359. Stillinger, F. H., Rahman, A. J. Chem. Phys. 1974; 60(4), 15451557. Matsuoka, O.; Clementi, E.; Yoshimine, M. J. Chem. Phys. 1976; 64, 1351-1361. Stillinger, F. H.; Rahman, A. J. Chem. Phys. 1978; 68(2), 666-670. Barnes, P.; Finney, J. L.; Nicholas, J.; Quinn, J. E. Nature 1979; 282, 459-464. Jorgensen, W. L.; Chandrasekhar, J.; Madura, J. D.; Impey, R. W.; Klein, M. L. J. Chem. Phys. 1983; 79, 926-935. Detrich, J.; Corongiu, G.; Clementi, E. IBM Res. Rept. 1984; KGN3, 1-11. Impey, R. W.; Madden, P. Α.; McDonald, I. R. Mol. Phys. 1982; 46(3), 513-539. van Gunsteren, W. F.; Berendsen, H. J. C.; Rullman, J. A. C. Faraday Disc. 1978; 66, 58-70. Stillinger, F. H.; Rahman, A. J. Chem. Phys. 1978; 68(2), 666-670. Rahman, Α.; Stillinger, F. H. J. Chem. Phys. 1971; 55(7), 33363359. Sonnenschein, R.; Heinzinger, K. Chem. Phys. Lett. 1983; 102(6), 550-554. Mulla, D. J.; Cushman, J. H.; Low, P. F. Water Resour. Res. 1984; 20(5), 619-628. Christou, Ν. I.; Whitehouse, J. S.; Nicholson, D.; Parsonage, N. G. Faraday Symp. Chem. Soc. 1981; 16, 139-149. Barabino, G.; Gavotti, C.; Marchesi, M. Chem. Phys. Lett. 1984; 104(5), 478-484. Hobza, P.; Sauer, J.: Morgeneyer, C.; Hurych, J.; Zahradnik, R. J. Phys. Chem. 1981; 85, 4061-4067. Geerlings, P.; Tariel, N.; Botrel, Α.; Lissillour, R.; Mortier, W. J. J. Phys. Chem. 1984; 88, 5752-5759. Sauer, J . ; Zahradnik, R. Intl. J. Quant. Chem. 1984; 26, 793-822. Low, P. F. Soil Sci. Soc. Am. J. 1981; 45(6), 1074-1078.

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Probst, M. M.; Radnai, T.; Heinzinger, K.; Bopp, P.; Rode, B. M. J. Phys. Chem. 1985; 89, 753-759. 42. Heinzinger, K. Pure Appl. Chem. 1985; 57(8), 1031-1042. 43. Bounds, D. G. Mol. Phys. 1985; 54(6), 1335-1355. 44. Gruen, D. W. R.; Marcelja, S.; Pailthorpe, B. A. Chem. Phys. Lett. 1981; 82(2), 315-320. 45. Jonsson, B. Chem. Phys. Lett. 1981; 82(3), 520-525. 46. Lee, C. Y.; McCammon, J. Α.; Rossky, P. J. J. Chem. Phys. 1984; 80(9), 4448-4445. 47. Marchesi, M. Chem. Phys. Lett. 1983; 97(2), 224-230. 48. Mulla, D. J.; Low, P. F.; Cushman, J. H.; Diestler, D. J. Colloid Interface Sci. 1984; 100(2), 576-580. 49. Pashley, R. M.; Israelachvi1i, J. N. J. Colloid Interface Sci. 1984; 101(2), 511-523. 50. Gruen, D. W. R.; Marcelja, S. J. Chem. Soc. Faraday Trans. 2 1983; 79, 225-242. 51. Goldman, S. J. Chem. Phys. 1981; 75(8), 4064-4076. 52. Steinhauser, O. Mol 53. Neumann, M. J. Chem 54. Helmy, A. K.; Natale, I. M. Clays Clay Min. 1985; 33(4), 329-332. 55. Fripiat, J. J.; Letellier, M.; Levitz, P. Phil. Trans. R. Soc. London 1984; A311, 287-299. 56. Margerum, D. W.; Cayley, G. R.; Weatherburn, D. C.; Pagenkopf, G. K. In "Coordination Chemistry"; Martell, A. E., Ed.; ACS Monograph 174, American Chemical Society: Washington, D. C., 1978; Vol. 2, Chap. 1. 57. Eigen, M. Angew. Chemie 1964; 3(1), 1-72. 58. Breen, C.; Adams, J.; Riekel, C. Clays Clay Min. 1985; 33(4), 275284. 59. Bassetti, V.; Burlamacchi, L.; Martini, G. J. Am. Chem. Soc. 1979; 101(19), 5471-5477. 60. Cruz, M. I.; Letellier, M.; Fripiat, J. J. J. Chem. Phys. 1978; 69(5), 2018-2027. 61. Martini, G. J. Colloid Interface Sci. 1981; 80(1), 39-48. 62. Roberts, Ν. K.; Zundel, G. J. Phys. Chem. 1980, 84, 3655-3660. 63. Mulla, D. J.; Low, P. F. J. Colloid Interface Sci. 1983; 95(1), 51-60. 64. Remerie, K.; van Gunsteren, W. F.; Postma, J. P. M.; Berendsen, H. J. C.; Engberts, J. B. F. N. Mol. Phys. 1984; 53(6), 1517-1526. 65. Kuharski, R. Α.; Rossky, P. J. J. Am. Chem. Soc. 1984; 106, 57865793. 66. Adelman, S. A. J. Phys. Chem. 1985; 89, 2213-2221. 67. Broughton, J. Q. Surface Sci. 1980; 91, 91-112. 68. Talbot, J.; Tildesley, D. J.; Steele, W. A. Mol. Phys. 1984; 51(6), 1331-1356. 69. Garofalini, S. H. J. Non-Crystalline Solids 1984; 63, 337-345. 70. Soules, T. F. J. Non-Crystalline Solids 1982; 49, 29-52. 71. Nose', S. Mol. Phys. 1984; 52(2), 255-268. 72. Ray, J. R.; Rahman, A. J. Chem. Phys. 1984; 80(9), 4423-4428. 73. Hoover, W. G.; Moran, B.; Haile, J. M. J. Stat. Phys. 1984; 37(1/2), 109-121. RECEIVED

June 18, 1986

In Geochemical Processes at Mineral Surfaces; Davis, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

3

Behavior o f Water o n the Surface o f K a o l i n M i n e r a l s R. F. Giese, Jr., and P. M. Costanzo Department of Geological Sciences, State University of New York at Buffalo, 4240 Ridge Lea Road, Amherst, NY 14226 Study of hydrated kaolinites shows that water molecules adsorbed on a phyllosilicate surface occupy two different structural sites One type of water "hole" water is keyed into th er, while the othe typ is situated between and is hydrogen bonded to the hole water molecules. In contrast, hole water is hydrogen bonded to the silicate layer and is less mobile than associated water. At low temperatures, all water molecules form an ordered structure reminiscent of ice; as the temperature increases, the associated water disorders progressively, culminating in a rapid change in heat capacity near 270 K. To the extent that the kaolinite surfaces resemble other silicate surfaces, hydrated kaolinites are useful models for water adsorbed on silicate minerals. To a large extent, the study of terrestrial geology is the study of the interaction of water and rock materials. Much of the modification of the earth s surface, involving chemical weathering, transport, and deposition of sediment, results from the contact of water, often containing reactive chemical species, with the surfaces of mineral grains. The majority of the chemical activity, as far as we presently know, takes place on a microscopic scale; at the interface between a mineral and the first, or perhaps the first few, layers of adsorbed water molecules. Such interfacial regions have complex physical chemical properties, often very different from the phases which they separate. This is compounded, in the case of the watersilicate interface, by the fact that the structure of bulk water is very complex itself, and, while we know in general terms the crystal structures of the major silicate minerals, we often do not have a clear picture of the structure of the mineral surface, nor do we know in detail the structure of disordered minerals, at least not on an atomic scale. The amount of water in the interfacial region is very small compared to the bulk water in the system. For many experimental 1

0097-6156/ 86/ 0323-0037S06.00/ 0 © 1986 American Chemical Society

In Geochemical Processes at Mineral Surfaces; Davis, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

G E O C H E M I C A L P R O C E S S E S AT M I N E R A L

38

SURFACES

t e c h n i q u e s , t h i s means t h a t the s i g n a l from the s u r f a c e water i s too weak t o be e a s i l y s e p a r a t e d from t h a t o f the b u l k w a t e r . One s o l u t i o n t o t h i s problem i s t o s t u d y m a t e r i a l s w i t h l a r g e s p e c i f i c s u r face areas. T r a d i t i o n a l l y , t h e s e have been c l a y m i n e r a l s , z e o l i t e s , and g e l s . Of t h e s e , c l a y m i n e r a l s a r e c o n c e p t u a l l y the s i m p l e s t because they p r e s e n t two d i m e n s i o n a l p l a n a r s u r f a c e s t o the e x t e r n a l environment. T h i s paper i s d i v i d e d i n t o two p a r t s ; an i n t r o d u c t i o n t o the p r o p e r t i e s o f water a t the m i n e r a l - w a t e r i n t e r f a c e , followed by a d e s c r i p t i o n o f the work done i n our l a b o r a t o r y on the s t r u c t u r e o f water i n the i n t e r l a y e r r e g i o n o f k a o l i n i t e w i t h p a r t i c u l a r r e f e r e n c e t o the s t r u c t u r e and p r o p e r t i e s o f water a t the i n t e r f a c e . Clay Mineral

Structures

The b a s i c s t r u c t u r e s o f the c l a y m i n e r a l s were d e s c r i b e d by P a u l i n g (J_, 2) and i l l u s t r a t i o n s o f each type c a n be found i n the t e x t o f Grim Q ) . These s t r u c t u r e models are based on r e g u l a r t e t r a h e d r a and o c t a h e d r a formed by oxygen o r h y d r o x y l g r o u p s , s y m m e t r i c a l l y disposed i n planar l a y e r s a c t u a l m i n e r a l s a r e f a r more complex (see (4.) f o r a r e c e n t summary). The two t y p e s o f c l a y m i n e r a l s t r u c t u r e s which a r e o f i n t e r e s t i n the p r e s e n t d i s c u s s i o n a r e the expanding 2:1 s t r u c t u r e s ( t h e s m e c t i t e s and v e r m i c u l i t e s ) and the 1:1 s t r u c t u r e s ( t h e k a o l i n s ) . The s m e c t i t e s and v e r m i c u l i t e s have a fundamental l a y e r made up o f two s h e e t s o f t e t r a h e d r a which i n c o r p o r a t e s m a l l , h i g h l y charged c a t i o n s and one s h e e t o f o c t a h e d r a c o o r d i n a t i n g l a r g e r c a t i o n s . The o c t a h e d r a share e d g e s , the t e t r a h e d r a share c o r n e r s , and the t h r e e s h e e t s s h a r e oxygens i n common p l a n e s t o form the 2:1 l a y e r ( F i g u r e 1A). A s i m i l a r scheme, but i n v o l v i n g o n l y one s h e e t o f t e t r a h e d r a and one o f o c t a h e d r a , produces the 1:1 l a y e r s i l i c a t e s ( F i g u r e 1B) o f which k a o l i n i t e i s perhaps the most i m p o r t a n t m i n e r a l . I n the s m e c t i t e s and v e r m i c u l i t e s , s u b s t i t u t i o n o f d i f f e r e n t l y charged i o n s i s common. These may i n v o l v e aluminum f o r s i l i c o n i n the t e t r a h e d r a l s i t e s , and, i n the o c t a h e d r a l s i t e s , f e r r o u s o r f e r r i c i r o n f o r aluminum, magnesium f o r aluminum, o r l i t h i u m f o r magnesium; i n a d d i t i o n , v a c a n t s i t e s a r e commonly f o u n d . The s u b s t i t u t i o n s c r e a t e a charge imbalance which i s n e u t r a l i z e d by the a d s o r p t i o n o f c a t i o n s on the e x t e r n a l and i n t e r n a l s u r f a c e s o f the crystals. These compensating c a t i o n s a r e n o t f i r m l y a t t a c h e d t o the c l a y s u r f a c e s and they c a n be exchanged by t r e a t m e n t w i t h d i l u t e s o l u t i o n s of appropriate s a l t s . When exposed t o water and many o r g a n i c m o l e c u l e s , the l a y e r s o f t h e s e m i n e r a l s s e p a r a t e a l l o w i n g the g u e s t m o l e c u l e s t o e n t e r between the l a y e r s . Thus, b o t h s u r f a c e s o f e v e r y l a y e r o f the c l a y c r y s t a l become e q u i v a l e n t t o e x t e r n a l s u r f a c e s and the t o t a l s u r f a c e a r e a i n c r e a s e s t o as much as 800 m /g i n the c a s e o f the s m e c t i t e s . The 1:1 k a o l i n s t r u c t u r e s a r e c h e m i c a l l y s i m p l e r ; the t e t r a h e d r a l s i t e s a r e o c c u p i e d by s i l i c o n and the o c t a h e d r a l s i t e s by aluminum. There i s a minor amount o f s u b s t i t u t i o n , l a r g e l y o f f e r r i c i r o n f o r aluminum, but the amounts a r e g e n e r a l l y o n l y a few t e n t h s o f a p e r c e n t by weight o f o x i d e . The k a o l i n m i n e r a l s do n o t expand i n the p r e s e n c e o f water and t h e i r s u r f a c e a r e a , a p p r o x i m a t e l y 10 t o 15 m / g , r e p r e s e n t s the e x t e r n a l a r e a o f the c r y s t a l s . Because o f the d i f f e r e n c e between the 2:1 and 1:1 s t r u c t u r e s , t h e i r e x t e r n a l and i n t e r n a l s u r f a c e s a r e f u n d a m e n t a l l y d i f f e r e n t .

In Geochemical Processes at Mineral Surfaces; Davis, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

3.

GIESE A N D COSTANZO

A

Water on the Surface of Kaolin

Minerals

39

Β

F i g u r e 1. P o l y h e d r a l r e p r e s e n t a t i o n s o f the l a y e r s i n 2:1 smec­ t i t e s (A) and 1:1 k a o l i n i t e s ( B ) . Only a s i n g l e l a y e r i s shown i n each. The view i s down t h e [041] a x i s o f the u n i t c e l l .

In Geochemical Processes at Mineral Surfaces; Davis, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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G E O C H E M I C A L P R O C E S S E S AT M I N E R A L S U R F A C E S

A l l s u r f a c e s o f t h e 2:1 c l a y s ( e x c l u d i n g adsorbed s p e c i e s ) c o n s i s t o f oxygen, w h i l e the 1:1 m i n e r a l s have one s u r f a c e o f each l a y e r formed by oxygen and the o t h e r s u r f a c e formed by h y d r o x y l g r o u p s . Hydration of Clavs. There i s g e n e r a l agreement, and ample e x p e r i m e n t a l and t h e o r e t i c a l e v i d e n c e t o s u p p o r t the p o i n t o f v i e w , t h a t w a t e r m o l e c u l e s i n c o n t a c t w i t h o r i n the c l o s e v i c i n i t y o f c l a y surfaces are perturbed. The d i s t a n c e o v e r which t h e s e p e r t u r b a t i o n s produce measurable changes i n the p r o p e r t i e s o f t h e adsorbed w a t e r m o l e c u l e s i s much more c o n t r o v e r s i a l . Two c o n f l i c t i n g models f o r w a t e r adsorbed on c l a y m i n e r a l s u r f a c e s have been p r o p o s e d ; one s t a t e s t h a t the p e r t u r b a t i o n by the s u r f a c e i s l i m i t e d t o between 3 and 4 water l a y e r s ( r o u g h l y 10A) (5.), w h i l e t h e o t h e r h o l d s t h a t the i n f l u e n c e o f the c l a y s u r f a c e s e x t e n d s much f u r t h e r , up t o 30 o r more water l a y e r s ( r o u g h l y 100A) (6,). E v a l u a t i n g the e v i d e n c e f o r e a c h model i s c o m p l i c a t e d because t h e y a r e based on d i f f e r e n t e x p e r i m e n t a l a p p r o a c h e s which probe t h e c l a y - w a t e r i n t e r a c t i o n s a t d i f f e r e n t time s c a l e s ( £ ) . the i n f l u e n c e o f t h e c l a o f t h e exchangeable c a t i o n s adsorbed on o r near t h e s u r f a c e . The f o l l o w i n g d i s c u s s i o n i s n o t meant t o be an e x h a u s t i v e e v a l u a t i o n o f t h e two m o d e l s , but r a t h e r a s e l e c t i v e d i s c u s s i o n o f what i s known about the s t r u c t u r e o f water i n c l o s e c o n t a c t w i t h a c l a y m i n e r a l s u r f a c e and what t h e g e n e r a l arguments a r e i n f a v o r o f o r a g a i n s t t h e s h o r t - r a n g e and l o n g - r a n g e i n t e r a c t i o n m o d e l s . More d e t a i l e d d i s c u s s i o n o f t h e problem c a n be found i n (2.) and (8.). Water on H a l l o v s i t e . C e n t r a l t o t h e c o n t r o v e r s y i s the o b s e r v a t i o n t h a t c l a y c r y s t a l s p r e s e n t a p l a n a r a r r a y o f oxygens (and h y d r o x y l s i n t h e c a s e o f k a o l i n i t e ) w h i c h have hexagonal ( o r n e a r l y ) symmetry w i t h a p e r i o d i c i t y s i m i l a r t o t h a t found i n the c r y s t a l s t r u c t u r e o f i c e . Because o f t h i s g e o m e t r i c s i m i l a r i t y , i t has f r e q u e n t l y been assumed t h a t water adsorbed on a c l a y s u r f a c e w i l l p r e f e r e n t i a l l y adopt an i c e - l i k e c o n f i g u r a t i o n . When l o o k e d a t i n d e t a i l , i t i s d i f f i c u l t to f i n d unequivocal evidence to support t h i s . H a l l o y s i t e - 1 0 A i s f r e q u e n t l y r e f e r e n c e d as an example o f w a t e r m o l e c u l e s w i t h an i c e - l i k e s t r u c t u r e , e p i t a x i a l l y adsorbed o n a c l a y s u r f a c e (2_). I n f a c t , t h i s model has been so a p p e a l i n g t h a t t h e o r i g i n a l f i g u r e i l l u s t r a t i n g t h e s t r u c t u r a l model o f H e n d r i c k s and J e f f e r s o n (5.) has been r e p r o d u c e d i n n u m e r a b l e t i m e s by o t h e r s ( s e e ( £ ) and (10.) f o r e x a m p l e ) . I t i s o f t e n s t a t e d i n the l i t e r a t u r e t h a t the H e n d r i c k s and J e f f e r s o n model i s based on X - r a y d i f f r a c t i o n d a t a , i m p l y i n g t h a t a s t r u c t u r e was proposed and t e s t e d by c o m p a r i son o f o b s e r v e d and c a l c u l a t e d i n t e n s i t i e s . What seems t o have been o v e r l o o k e d i s the f a c t t h a t the o r i g i n a l paper o f H e n d r i c k s and J e f f e r s o n p r e s e n t e d no s u b s t a n t i a l e x p e r i m e n t a l e v i d e n c e t o s u p p o r t t h e i r model o t h e r t h a n the o b s e r v e d i n c r e a s e i n the t h i c k n e s s o f t h e c l a y s t r u c t u r e r e s u l t i n g from h y d r a t i o n . That d i s t a n c e , r o u g h l y 2 . 9 A, does impose some g e o m e t r i c r e s t r i c t i o n on the p o s s i b l e a r r a n g e ment o f the i n t e r l a y e r water m o l e c u l e s , b u t h a r d l y a d e f i n i t i v e o n e . Much subsequent work has shown t h a t i t i s v e r y d i f f i c u l t t o o b t a i n good agreement between o b s e r v e d and c a l c u l a t e d i n t e n s i t i e s even f o r the s i m p l e c a s e o f the o n e - d i m e n s i o n a l s t r u c t u r e a l o n g the £ - a x i s which i n v o l v e s o n l y the & c o o r d i n a t e o f t h e atoms (see (11) for discussion).

In Geochemical Processes at Mineral Surfaces; Davis, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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Water on the Surface of Kaolin

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H a l l o y s i t e - 1 0 A r e p r e s e n t s a s t r u c t u r e w i t h few i f any i n t e r l a y e r c a t i o n s , a l l o w i n g one t o i n v e s t i g a t e t h e r e l a t i v e l y s i m p l e c a s e o f water i n t e r a c t i n g w i t h a c l a y s u r f a c e . Similarly, ice-like models have been proposed f o r water adsorbed on s m e c t i t e and v e r m i c u l i t e s u r f a c e s ( £ , J_2, 12.). These r e p r e s e n t c a s e s o f charged c l a y l a y e r s w i t h adsorbed exchangeable c a t i o n s . Water on V e r m i c u l i t e . F o r low water c o n t e n t s ( t h a t i s , one o r two water l a y e r s ) , the e v i d e n c e f o r h i g h l y s t r u c t u r e d water i n t h e i n t e r l a y e r s p a c e s o f s m e c t i t e s and v e r m i c u l i t e s i s most e a s i l y s e e n i n X-ray d i f f r a c t i o n s t r u c t u r e determinations of ordered hydrate s t r u c t u r e s s u c h as the two-water l a y e r h y d r a t e o f C a - v e r m i c u l i t e (11. 15.) and N a - v e r m i c u l i t e (15., 16). I n the C a - v e r m i c u l i t e , the i n t e r l a y e r c a l c i u m i o n s a r e o f two t y p e s ; one w i t h s i x water m o l e c u l e s i n an o c t a h e d r a l arrangement, the o t h e r w i t h e i g h t water m o l e c u l e s a r r a n g e d i n a d i s t o r t e d c u b e . The v e r m i c u l i t e l a y e r s a r e s t a c k e d so t h a t the d i t r i g o n a l h o l e s i n the t e t r a h e d r a l s u r f a c e hedral s i t e s . The s i x - c o o r d i n a t e t e t r a h e d r a where t h e y c a n most e f f e c t i v e l y compensate t h e c h a r g e d e f i c i e n c y r e s u l t i n g from t h e aluminum f o r s i l i c o n s u b s t i t u t i o n . In c o n t r a s t , t h e e i g h t c o o r d i n a t e d c a l c i u m i o n s a r e p o s i t i o n e d between the d i t r i g o n a l h o l e s , a p o s i t i o n where t h e y a r e a l s o v e r y c l o s e t o the t e t r a h e d r a l c h a r g e d e f i c i t s , the c a t i o n s i n t h e N a - v e r m i c u l i t e a r e a l l o c t a h e d r a l l y c o o r d i n a t e d by w a t e r m o l e c u l e s and l i e e x c l u s i v e l y between t e t r a h e d r a l oxygens o f t h e a d j a c e n t l a y e r s . Water on S m e c t i t e s . Compared t o v e r m i c u l i t e s , s m e c t i t e s p r e s e n t a more d i f f i c u l t e x p e r i m e n t a l system because o f t h e l a c k o f s t a c k i n g o r d e r o f the l a y e r s . F o r t h e s e m a t e r i a l s , the t r a d i t i o n a l t e c h n i q u e o f X - r a y d i f f r a c t i o n , e i t h e r u s i n g the Bragg o r n o n - B r a g g i n t e n s i t i e s , i s of l i t t l e use. Spectroscopic techniques, e s p e c i a l l y nuclea r magnetic r e s o n a n c e and i n f r a r e d , as w e l l as n e u t r o n and X - r a y s c a t t e r i n g have p r o v i d e d d e t a i l e d i n f o r m a t i o n about the p o s i t i o n o f the water m o l e c u l e s , the dynamics o f the water m o l e c u l e m o t i o n s , and the c o o r d i n a t i o n about the i n t e r l a y e r c a t i o n s . As an example, i n f r a r e d s p e c t r o s c o p y has shown t h a t the l o w e s t s t a b l e h y d r a t i o n s t a t e f o r a L i - h e c t o r i t e has a s t r u c t u r e i n which the l i t h i u m c a t i o n i s p a r t i a l l y keyed i n t o t h e d i t r i g o n a l h o l e o f t h e h e c t o r i t e and has 3 water m o l e c u l e s c o o r d i n a t i n g the exposed p a r t o f t h e c a t i o n i n a t r i a n g u l a r arrangement (17 ), as proposed i n t h e model o f Mamy ( 1 3 ) . The water m o l e c u l e s e x h i b i t two k i n d s o f m o t i o n ; a slow r o t a t i o n o f the whole h y d r a t i o n s p h e r e about an a x i s t h r o u g h the t r i a n g l e o f t h e water m o l e c u l e s , and a f a s t e r r o t a t i o n o f each water m o l e c u l e about i t s own Cp a x i s (18). A similar s t r u c t u r e f o r adsorbed water a t low water c o n t e n t s has been o b s e r v e d f o r C u - h e c t o r i t e , C a - b e n t o n i t e , and C a - v e r m i c u l i t e ( 1 7 ) . M u l t i l a y e r A d s o r p t i o n o f Water. As the amount o f w a t e r i n the c l a y i n c r e a s e s o v e r t h a t needed f o r a o n e - o r t w o - l a y e r h y d r a t e , t h e s t u d y o f the p r o p e r t i e s o f the water becomes e x p e r i m e n t a l l y more difficult. T h i s i s i m p o r t a n t because i t i s o n l y a t water c o n t e n t s i n e x c e s s o f the t w o - l a y e r h y d r a t e t h a t a c o n f l i c t a r i s e s between t h e s h o r t - r a n g e and l o n g - r a n g e i n t e r a c t i o n models. In support o f t h e s h o r t - r a n g e model, two s t u d i e s a r e n o t e w o r t h y . A small angle

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X - r a y d i f f r a c t i o n s t u d y o f c l a y - w a t e r g e l s (19) has shown t h a t t h e r e a r e s t r o n g i n t e r a c t i o n s between the c l a y s u r f a c e s and t h e i r adsorbed i o n s and water m o l e c u l e s , but t h e s e i n t e r a c t i o n s e x i s t o n l y o v e r short distances. An e n t i r e l y d i f f e r e n t approach has been t a k e n by F r i p i a t e t a l . (5.). They examined c l a y - w a t e r m i x t u r e s o v e r a wide range o f c l a y c o n c e n t r a t i o n s u s i n g k a o l i n i t e and s m e c t i t e s . For m i c r o s c o p i c water p r o p e r t i e s , the n u c l e a r magnetic r e s o n a n c e r e l a x a t i o n time f o r hydrogen and d e u t e r i u m were measured. The m a c r o s c o p i c p r o p e r t i e s were d e t e r m i n e d by m e a s u r i n g the h e a t s o f w e t t i n g o f previously equilibrated clay-water mixtures. Both e x p e r i m e n t a l t e c h n i q u e s , o p e r a t i n g on v e r y d i f f e r e n t time s c a l e s , showed t h a t the number o f water l a y e r s i n f l u e n c e d by t h e c l a y was l e s s t h a n 5, w i t h an average f o r a l l samples o f 3.4 l a y e r s . T h i s g i v e s an average t h i c k n e s s o f about 10 A f o r t h e p e r t u r b e d w a t e r . The view t h a t the c l a y s u r f a c e p e r t u r b s water m o l e c u l e s a t d i s t a n c e s w e l l i n e x c e s s o f 10 A has been l a r g e l y based on measurements o f thermodynamic p r o p e r t i e s o f t h e adsorbed w a t e r as a f u n c t i o n o f the water c o n t e n e x t e n s i v e l i t e r a t u r e on Low (6_). The p r o p e r t i e s examined a r e , among o t h e r s , the a p p a r e n t s p e c i f i c heat c a p a c i t y , the p a r t i a l s p e c i f i c volume, and the a p p a r e n t s p e c i f i c e x p a n s i b i l i t y (6_). These measurements were made on samples p r e p a r e d by m i x i n g p r e d e t e r m i n e d amounts o f water and smect i t e t o a c h i e v e t h e d e s i r e d number o f adsorbed w a t e r l a y e r s . The number o f water l a y e r s adsorbed on t h e c l a y i s d e r i v e d from the amount o f water added t o the c l a y and the s u r f a c e a r e a o f the c l a y . The v a l u e o f the thermodynamic p r o p e r t y i n q u e s t i o n i s the d i f f e r e n c e i n v a l u e s f o r the c l a y - w a t e r sample and the same measurement on an e q u i v a l e n t amount o f p u r e , anhydrous c l a y (6.). This procedure i n v o l v e s two a s s u m p t i o n s : 1) t h e added water i s u n i f o r m l y adsorbed on a l l c l a y l a y e r s , and 2) the thermodynamic p r o p e r t i e s o f the c l a y i t s e l f do n o t change when the c l a y expands and i s i n t e r c a l a t e d by water m o l e c u l e s . The a s s u m p t i o n t h a t t h e water i s adsorbed i n u n i f o r m l a y e r s on a l l t h e c l a y s u r f a c e s f o r a wide r a n g e o f m i x t u r e s has been c r i t i c i z e d (5., 20 ). The argument i s t h a t the i n d i v i d u a l c l a y p a r t i c l e s i n t h e c l a y - w a t e r m i x t u r e do n o t expand beyond a c e r t a i n d i s t a n c e r e g a r d l e s s o f the q u a n t i t y o f water which i s added. The c l a y l a y e r s group t h e m s e l v e s i n t o t a c t o i d s r e s u l t i n g i n two p o p u l a t i o n s o f w a t e r ; t h o s e m o l e c u l e s which a r e found between the t a c t o i d s and t h o s e d i r e c t l y p e r t u r b e d by t h e c l a y l a y e r s . I f t r u e , t h i s would i n v a l i d a t e the p r o c e d u r e used t o c a l c u l a t e the thermodynamic p r o p e r t i e s o f the adsorbed w a t e r . However, o t h e r workers have r e p o r t e d c o m p l e t e d e l a m i n a t i o n o f c e r t a i n s m e c t i t e s (21, 2 2 ) . I t i s not c l e a r under what c o n d i t i o n s t a c t o i d s w i l l f o r m , o r n o t , and t h i s u n c e r t a i n t y i s u n d e r l i n e d i n (21) (see remarks by Nadeau and F r i p i a t , pages 1 4 6 - 1 4 7 ) . The v a l i d i t y o f the a s s u m p t i o n t h a t the v a r i o u s thermodynamic p r o p e r t i e s o f the s m e c t i t e remain i n v a r i a n t , r e g a r d l e s s o f t h e s t a t e o f h y d r a t i o n , has been a d d r e s s e d i n d e t a i l by S p o s i t o and P r o s t (JJ · They p o i n t o u t t h a t one would, f o r example, e x p e c t h y d r o l y s i s o f t h e c l a y t o o c c u r a t h i g h w a t e r c o n t e n t s , and a l s o , i t i s l i k e l y t h a t t h e exchangeable c a t i o n s w i l l change t h e i r s p a t i a l r e l a t i o n s h i p w i t h the c l a y l a y e r s . T h u s , the d e r i v e d thermodynamic p r o p e r t i e s o f the adsorbed water would not r e p r e s e n t c o r r e c t v a l u e s .

In Geochemical Processes at Mineral Surfaces; Davis, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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F i n a l l y , the whole c o n c e p t o f u s i n g m a c r o s c o p i c ( i . e . thermodynamic) p r o p e r t i e s t o d e r i v e a m i c r o s c o p i c p i c t u r e o f the adsorbed water i s open t o q u e s t i o n ( £ , 8.). I t i s d i f f i c u l t t o r e c o n c i l e t h e s e v e r y d i f f e r e n t views o f the i n t e r a c t i o n o f water and c l a y s u r f a c e s . S p o s i t o (8.) has attempted this. He p o i n t s o u t t h a t the thermodynamic p r o p e r t i e s have an e s s e n t i a l l y i n f i n i t e time s c a l e , whereas the s p e c t r o s c o p i c measurements l o o k a t some v a r i a n t o f the v i b r a t i o n a l o r a p r e d e c e s s o r o f the d i f f u s i o n a l s t r u c t u r e o f w a t e r . I t i s p o s s i b l e t h a t the thermodynamic p r o p e r t i e s r e f l e c t a number o f c o o p e r a t i v e i n t e r a c t i o n s which c a n be seen o n l y on a v e r y l o n g time s c a l e . S t i l l , the X - r a y d i f f r a c t i o n s t u d i e s s e e m i n g l y a l s o o p e r a t e on as l o n g a time s c a l e as the thermodynamic p r o p e r t i e s . There i s s t i l l n o t a c l e a r c h o i c e between the s h o r t - r a n g e and l o n g - r a n g e i n t e r a c t i o n m o d e l s . E x p e r i m e n t a l S t u d i e s o f Water on K a o l i n M i n e r a l s I n g e n e r a l , the 2:1 c l a y s t u d y the i n t e r a c t i o n o v a r i a b l e c o m p o s i t i o n s and t h e i r s t r u c t u r e s a r e p o o r l y u n d e r s t o o d . Water o c c u r s i n s e v e r a l d i f f e r e n t e n v i r o n m e n t s : z e o l i t i c water i n the i n t e r l a y e r r e g i o n s , water adsorbed on t h e e x t e r n a l s u r f a c e s o f the c r y s t a l l i t e s , water c o o r d i n a t i n g the exchangeable c a t i o n s , a n d , o f t e n , as pore water f i l l i n g v o i d s between t h e c r y s t a l l i t e s . Thus, t h e r e a r e many v a r i a b l e s and the e f f e c t s o f each on t h e p r o p e r t i e s o f water a r e d i f f i c u l t t o s e p a r a t e . I n view o f the problems a s s o c i a t e d w i t h the expanding 2:1 c l a y s , the s m e c t i t e s and v e r m i c u l i t e s , i t seemed d e s i r a b l e t o use a d i f f e r e n t c l a y m i n e r a l s y s t e m , one i n which the i n t e r a c t i o n s o f s u r f a c e adsorbed water a r e more e a s i l y s t u d i e d . An o b v i o u s c a n d i d a t e i s t h e h y d r a t e d form o f h a l l o y s i t e , but s t u d i e s o f t h i s m i n e r a l have shown t h a t h a l l o y s i t e s a l s o s u f f e r from an e q u a l l y i n t r a c t a b l e s e t o f d i f f i c u l t i e s (J_0). These a r e p r i n c i p a l l y the poor c r y s t a l l i n i t y , the n e c e s s i t y t o m a i n t a i n the c l a y i n l i q u i d water i n o r d e r t o p r e v e n t l o s s o f the s u r f a c e adsorbed ( i n t e r c a l a t e d ) w a t e r , and the h i g h l y v a r i a b l e morphology o f the c r y s t a l l i t e s . I t seemed t o u s p r e f e r a b l e t o s t a r t w i t h a c h e m i c a l l y p u r e , w e l l - c r y s t a l l i z e d , and w e l l - k n o w n c l a y m i n e r a l ( k a o l i n i t e ) and t o i n c r e a s e t h e n o r m a l l y s m a l l s u r f a c e a r e a by i n s e r t i n g water m o l e c u l e s between the l a y e r s through chemical treatment. Thus, the water would be i n c o n t a c t w i t h b o t h s u r f a c e s o f e v e r y c l a y l a y e r i n the c r y s t a l l i t e s r e s u l t i n g i n an e f f e c t i v e s u r f a c e a r e a f o r water a d s o r p t i o n o f a p p r o x i m a t e l y 1000 m g . The s y n t h e t i c k a o l i n i t e h y d r a t e s t h a t r e s u l t e d from t h i s work a r e n e a r l y i d e a l m a t e r i a l s f o r s t u d i e s o f water adsorbed on s i l i c a t e s u r f a c e s . Kaolin Minerals. The 1:1 s t r u c t u r e s i n c l u d e a group o f a l u m i n o s i l i c a t e m i n e r a l s which a r e termed c o l l e c t i v e l y the k a o l i n m i n e r a l s ; s p e c i f i c a l l y t h e s e a r e k a o l i n i t e , d i c k i t e , n a c r i t e , and h a l l o y s i t e . The b a s i c 1:1 l a y e r f o r a l l o f t h e s e m i n e r a l s has t h e c o m p o s i t i o n A l S i 0 ( 0 H ) . ; t h e r e i s a s m a l l amount o f s u b s t i t u t i o n o f i r o n f o r aluminum, ana f l u o r i d e f o r h y d r o x y l i o n . A l l , except h a l l o y s i t e , a r e n o r m a l l y anhydrous and do not expand (as do the s m e c t i t e s ) upon exposure t o water and most o r g a n i c m o l e c u l e s . As a r e s u l t , t h e y ^ g e n e r a l l y have a r a t h e r s m a l l s u r f a c e a r e a , on the o r d e r o f 10 m 2

2

5

]

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g . I n s p i t e o f t h e i r r e l a t i v e l y s i m p l e c o m p o s i t i o n and the abundance o f w e l l c h a r a c t e r i z e d s a m p l e s , the s m a l l s u r f a c e a r e a o f k a o l i n i t e and d i c k i t e h a s , u n t i l r e c e n t l y , e f f e c t i v e l y e l i m i n a t e d them as c a n d i d a t e s f o r s u r f a c e c h e m i s t r y s t u d i e s . Assuming l i t t l e o r no i s o m o r p h i c s u b s t i t u t i o n , the b o n d i n g between l a y e r s i s l a r g e l y due t o hydrogen bonds from the h y d r o x y l s o f one s u r f a c e t o a d j a c e n t oxygens o f the next s u r f a c e (22, 2 4 ) . Hydrogen bonds a r e n o t n o r m a l l y thought o f as b e i n g v e r y s t r o n g r e l a t i v e t o i o n i c and c o v a l e n t bonds, and one m i g h t , t h e r e f o r e , e x p e c t t h a t t h e l a y e r s o f t h e k a o l i n m i n e r a l s would be e a s i l y s e p a r a t e d , a t l e a s t by s m a l l m o l e c u l e s and w a t e r . Such i s not the c a s e f o r w a t e r , and i n t e r c a l a t i o n i s found t o be p o s s i b l e o n l y f o r a r e l a t i v e l y s m a l l number o f o r g a n i c m o l e c u l e s and s a l t s . The t y p e s o f o r g a n i c m o l e c u l e s t h a t a r e a b l e t o i n t e r c a l a t e the k a o l i n m i n e r a l s a r e g e n e r a l l y s m a l l w i t h l a r g e d i p o l e moments. These i n c l u d e h y d r a z i n e , d i m e t h y l s u l f o x i d e (DMSO), formamide and some d e r i v a t i v e s (N-methylformamide and d i m e t h y l f o r m a m i d e ) , acetamide and some d e r i v a t i v e s as p o t a s s i u m a c e t a t e a l s by one o f t h e s e s m a l l m o l e c u l e s o r s a l t s , o t h e r m o l e c u l e s w h i c h n o r m a l l y do n o t d i r e c t l y i n t e r c a l a t e k a o l i n s c a n be i n t r o d u c e d by replacement. F u r t h e r , the e x p o s u r e o f t h e i n n e r s u r f a c e s by i n t e r c a l a t i o n g i v e s one the o p p o r t u n i t y t o a l t e r the i n t e r l a y e r b o n d i n g o f t h e k a o l i n l a y e r s by c h e m i c a l m o d i f i c a t i o n o f the i n n e r s u r f a c e s . Synthesis of K a o l i n i t e Hydrates. Our work i n s y n t h e s i z i n g a w a t e r s i l i c a t e system t h a t has a s i m p l e r c h e m i s t r y t h a n the s m e c t i t e s o r v e r m i c u l i t e s i s based on the c o n c e p t o f r e d u c i n g the t o t a l number o f i n t e r l a y e r hydrogen bonds by a c h e m i c a l r e p l a c e m e n t o f some i n n e r s u r f a c e h y d r o x y l s by f l u o r i n e (25.). I n p r i n c i p l e , one need o n l y expand t h e k a o l i n i t e by i n t e r c a l a t i o n w i t h an a p p r o p r i a t e o r g a n i c m o l e c u l e and t h e n expose the i n t e r c a l a t e d c l a y t o an environment containing fluoride ions. In p r a c t i c e , t h e o r g a n i c m o l e c u l e , t h e t y p e o f f l u o r i d e s a l t added, as w e l l as the time and t e m p e r a t u r e a t e a c h s t a g e i n the development o f a h y d r a t e d k a o l i n i t e , p l a y an i m p o r t a n t r o l e i n d e t e r m i n i n g the s u c c e s s o f the s y n t h e s i s and the y i e l d of hydrated c l a y . E a r l y work showed t h a t a 10A h y d r a t e , s i m i l a r t o n a t u r a l l y h y d r a t e d h a l l o y s i t e , c o u l d be s y n t h e s i z e d from a w e l l - c r y s t a l l i z e d k a o l i n i t e from C o r n w a l l , E n g l a n d (26.). The p r o c e d u r e was t o i n t e r c a l a t e the c l a y w i t h DMSO which c o n t a i n e d about 8% by weight o f water. The c l a y expanded from 7 . 2 A t o 11 A upon i n t e r c a l a t i o n by DMSO. Ammonium f l u o r i d e was d i s s o l v e d i n the DMSO s o l u t i o n a n d , p r e s u m a b l y , the f l u o r i d e d i f f u s e d i n t o t h e i n t e r l a y e r r e g i o n where i t r e p l a c e d some h y d r o x y l g r o u p s . T h i s s u s p e n s i o n was c o n t i n u o u s l y s t i r r e d a t 6 0 C f o r p e r i o d s v a r y i n g from a few h o u r s t o as much as 12 h o u r s o r more. The c l a y was t h e n s e p a r a t e d from the water-DMSO by c e n t r i f u g a t i o n and r e d i s p e r s e d i n d i s t i l l e d w a t e r . This cycle of water washing was r e p e a t e d s e v e r a l t i m e s t o remove i n t e r c a l a t e d DMSO molecules. I n s t e a d o f c o l l a p s i n g back t o 7 A, t h e k a o l i n i t e e x h i b i t e d a 10 A s p a c i n g s i m i l a r t o t h a t o f a f u l l y h y d r a t e d h a l l o y s i t e - 1 0 A . I n f r a r e d s p e c t r a showed a broad a b s o r p t i o n i n the 3 4 0 0 3500 cm" r e g i o n and a s i n g l e band near 1650 cm i n d i c a t i n g that t h e k a o l i n i t e was i n t e r c a l a t e d p r i m a r i l y by w a t e r . As i s t h e c a s e w i t h h y d r a t e d h a l l o y s i t e s , t h i s 10A p r o d u c t was not s t a b l e under e

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ambient h u m i d i t y and t h u s was o f l i m i t e d use i n e x a m i n i n g the p r o p e r t i e s o f s u r f a c e adsorbed w a t e r . Subsequent work showed t h a t a m o d i f i c a t i o n o f t h e s y n t h e s i s p r o c e d u r e produced a 10A h y d r a t e w h i c h , i f d r i e d c a r e f u l l y , would m a i n t a i n the i n t e r l a y e r water i n the absence o f e x c e s s w a t e r ( 2 1 ) . T h i s m a t e r i a l i s o p t i m a l f o r adsorbed water s t u d i e s f o r a number o f reasons: the parent c l a y i s a w e l l - c r y s t a l l i z e d k a o l i n i t e w i t h a n e g l i g i b l e l a y e r c h a r g e , t h e r e a r e few i f any i n t e r l a y e r c a t i o n s , t h e r e i s no i n t e r f e r e n c e from pore water s i n c e t h e amount i s m i n i m a l , and the i n t e r l a y e r water m o l e c u l e s l i e between u n i f o r m l a y e r s o f known s t r u c t u r e . T h u s , t h e h y d r a t e p r o v i d e s a u s e f u l model f o r s t u d y i n g the e f f e c t s o f a s i l i c a t e s u r f a c e on i n t e r l a y e r w a t e r . C h a r a c t e r i z a t i o n o f I n t e r l a v e r Water. X-ray d i f f r a c t i o n studies of the 10A h y d r a t e show no h k l r e f l e c t i o n s i n d i c a t i n g a l a c k o f r e g u l a r i t y i n the s t a c k i n g o f the k a o l i n l a y e r s . In a d d i t i o n to the 10A h y d r a t e , two o t h e r l e s s h y d r a t e d k a o l i n i t e s were s y n t h e s i z e d . B o t h have one m o l e c u l e o the 10A h y d r a t e which ha q u e n t l y have s m a l l e r d(001) s p a c i n g s o f 8.4 and 8.6 A. The s y n t h e s i s c o n d i t i o n s f o r t h e s e two h y d r a t e s a r e d e s c r i b e d i n ( 2 £ ) . By s t u d y i n g the i n t e r l a y e r water i n the 8.4 and 8.6A h y d r a t e s , i t was p o s s i b l e t o f o r m u l a t e a model o f the w a t e r i n the more c o m p l i c a t e d 10A h y d r a t e . An i s o l a t e d h y d r o x y l g r o u p , s u c h as i s found i n the 2:1 m i c a s , a b s o r b s i n f r a r e d r a d i a t i o n a t a p p r o x i m a t e l y 3700 cm (28). Kaol i n i t e i s more complex s i n c e t h e r e a r e two d i s t i n c t t y p e s o f h y d r o x y l g r o u p s , the s i n g l e h y d r o x y l a t the i n t e r f a c e between the t e t r a h e d r a l and o c t a h e d r a l s h e e t s ( t h e i n n e r h y d r o x y l ) , and t h r e e h y d r o x y l s ( t h e i n n e r s u r f a c e h y d r o x y l s ) which form one o f t h e e x t e r n a l s u r f a c e s o f the k a o l i n i t e l a y e r . Because the i n n e r h y d r o x y l i s i s o l a t e d i t p r o d u c e s a s i n g l e v i b r a t i o n a t a p p r o x i m a t e l y 3620 cm" . The t h r e e e x t e r n a l h y d r o x y l s do n o t v i b r a t e i n d e p e n d e n t l y ; t h e i r s t r e t c h i n g v i b r a t i o n s a r e c o u p l e d t o produce s e v e r a l bands, none o f which c a n be r e l a t e d t o a s p e c i f i c h y d r o x y l group (22., 1 0 ) . T h e r e a r e t h r e e o f t h e s e bands f o r k a o l i n i t e ; a s t r o n g a b s o r p t i o n a t 3695 cm" , and two much weaker a b s o r p t i o n s a t 3665 and 3650 c m . The f o u r bands may v a r y i n f r e q u e n c y and i n t e n s i t y o f a b s o r p t i o n from sample t o sample. I n c o n t r a s t , the v i b r a t i o n a l bands o f water m o l e c u l e s i n t h e s o l i d phase o c c u r a t 3350 and 3250 cm" w i t h a b e n d i n g mode a t 1640 cm" ( 2 1 , 2 £ ) . Hence the i n f r a r e d bands from t h e s t r u c t u r a l h y d r o x y l s and i n t e r c a l a t e d water do n o t o v e r l a p . Of i m p o r t a n c e t o t h e p r e s e n t s u b j e c t i s the f a c t t h a t t h e t h r e e bands c o r r e s p o n d i n g t o the i n n e r s u r f a c e h y d r o x y l s c a n be used t o i n f e r the b o n d i n g s t a t e o f m o l e c u l e s i n the i n t e r l a y e r r e g i o n o f t h e kaolinite. I n s e r t i o n o f g u e s t m o l e c u l e s between t h e k a o l i n i t e l a y e r s d i s r u p t s the hydrogen bonds o f the o r i g i n a l k a o l i n i t e and r e s u l t s i n the f o r m a t i o n o f new bonds w i t h the g u e s t m o l e c u l e s . To r e c o r d the i n f r a r e d s p e c t r a , samples o f the p a r e n t k a o l i n i t e and the t h r e e h y d r a t e s were d i s p e r s e d i n a f l u o r i n a t e d h y d r o c a r b o n . The m u l l s were squeezed between c a l c i u m f l u o r i d e p l a t e s and t h e sample was p l a c e d d i r e c t l y i n the beam o f a P e r k i n - E l m e r 683 s p e c trometer. T h i s mounting t e c h n i q u e r e s u l t s i n a tendency f o r t h e c l a y l a y e r s t o a l i g n t h e m s e l v e s p e r p e n d i c u l a r t o t h e beam o f t h e spectrometer. I n f r a r e d s p e c t r a o f t h e s e m a t e r i a l s have been p u b -

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l i s h e d e l s e w h e r e (22.) and t h e r e a r e s l i g h t d i f f e r e n c e s i n f r e q u e n c i e s ( 2 - 3 cm"" o r l e s s ) and band i n t e n s i t i e s compared t o the r e s u l t s reported here. There a r e s e v e r a l r e a s o n s f o r t h e s e d i f f e r e n c e s : for t h i s work, we used f r e s h l y p r e p a r e d h y d r a t e s which seem t o g i v e a b e t t e r s i g n a l , t h e sample p r e p a r a t i o n f o r the i n f r a r e d s p e c t r o m e t e r has improved o v e r our o r i g i n a l work, and the p r e s e n t s p e c t r a were run i n the absorbance r a t h e r t h a n i n the t r a n s m i s s i o n mode. The s p e c t r a o f the h y d r a t e d k a o l i n i t e s ( F i g u r e 2A, 2B, 2C) d i f f e r s u b s t a n t i a l l y from e a c h o t h e r and a l s o from t h e p a r e n t k a o l i n i t e ( F i g u r e 2D). The 8.4A h y d r a t e ( F i g u r e 2A) has an o r d e r e d l a y e r s t a c k i n g , as shown by X - r a y d i f f r a c t i o n ( H ) . The c r y s t a l s t r u c t u r e shows t h a t the water m o l e c u l e s a r e a s s o c i a t e d w i t h and a r e keyed i n t o the d i t r i g o n a l h o l e s of the t e t r a h e d r a . The c o n d i t i o n s o f the 8.4A s y n t h e s i s r e s u l t i n up t o 20% o f t h e i n n e r s u r f a c e h y d r o x y l s b e i n g r e p l a c e d by f l u o r i d e i o n s . Because i t seems l i k e l y t h a t some o f the i n t e r l a y e r water m o l e c u l e s a r e hydrogen bonded t o f l u o r i d e i o n s o f t h e a d j a c e n t l a y e r , a t l e a s t two hydrogen bond schemes c a n be e n v i s i o n e d : one wher t e t r a h e d r a l s h e e t ; the o t h e one l a y e r and a f l u o r i d e o f t h e o t h e r l a y e r ( o r , p o s s i b l y , t o two f l u o r i d e s o f t h e same l a y e r ) . Hydrogen bonds t o f l u o r i d e a r e n o r m a l l y s t r o n g e r t h a n s i m i l a r bonds t o oxygen so one would e x p e c t t o see s e p a r a t e a b s o r p t i o n bands i n the i n f r a r e d spectrum o f t h e 8.4A h y d r a t e i f t h e r e a r e s u b s t a n t i a l numbers o f water m o l e c u l e s b o n d i n g to f l u o r i d e (27). I n c o n t r a s t t o the 8.4A h y d r a t e , t h e 8.6A h y d r a t e ( F i g u r e 2B) has o n l y a minor amount o f r e p l a c e m e n t o f f l u o r i d e f o r h y d r o x y l i o n s and a n o n - c r y s t a l l i n e c h a r a c t e r , l i k e the 10A h y d r a t e . Comparison o f the i n f r a r e d s p e c t r a o f the 8.4 and 8.6A h y d r a t e s ( F i g u r e s 2A, 2B) does i n d e e d show more bands f o r t h e former i n t h e r e g i o n between 3600 and 3200 c m , i n agreement w i t h t h e argument s t a t e d above. In t h e b e n d i n g mode r e g i o n , the 8.4A h y d r a t e has two c l e a r l y s e p a r a t e d bands, as one would e x p e c t f o r water hydrogen bonded t o oxygen (1645 c m " ) and a l s o hydrogen bonded t o f l u o r i d e (1590 c m ) . In c o n t r a s t , t h e 8.6A h y d r a t e has a s i n g l e band a t 1653 cm" . 1

Because the i n n e r h y d r o x y l i s b u r i e d i n the k a o l i n i t e l a y e r and i s n o t p e r t u r b e d s u b s t a n t i a l l y by i n t e r c a l a t i o n , i t s v i b r a t i o n , a t 3620 cm" , i s common t o a l l f o u r c l a y s i n F i g u r e 2. The 8.4A h y d r a t e has two h i g h f r e q u e n c y bands above 3620 cm" ; 3690 and 3650 c m . S i m i l a r bands a r e e v i d e n t i n t h e 8.6A h y d r a t e a l t h o u g h t h e i r i n t e n s i t i e s a r e somewhat d i f f e r e n t . Below 3620 c m , the 8.4A h y d r a t e has s m a l l bands a t 3584 and 3538 cm" and l a r g e r bands a t 3443 and 3340 cm" . The l a t t e r two do n o t appear i n the s p e c t r u m o f t h e 8.6A h y d r a t e and have been a s s i g n e d t o hydrogen b o n d i n g from water m o l e c u l e s t o f l u o r i d e . Bands s i m i l a r t o the 3584 and 3538 cm bands o f the 8.4A h y d r a t e appear i n t h e 8.6A h y d r a t e s p e c t r u m , b u t s h i f t e d s l i g h t l y t o 3595 and 3547 c m . The c l o s e match between t h e s e comparable bands a t f r e q u e n c i e s above 3500 cm" i n the two h y d r a t e s i s good e v i d e n c e t h a t t h e s e a r e due t o h y d r o x y l s and w a t e r molecules i n s i m i l a r environments. The water m o l e c u l e s a t t a c h e d t o t h e d i t r i g o n a l h o l e s have been termed " h o l e w a t e r " (22.). These r e p r e s e n t a d i s c o n t i n u o u s monolayer o f water adsorbed o n t o a s i l i c a t e s u r f a c e (33.). Weight l o s s measurements (22.) show t h a t t h e r e a r e as many water m o l e c u l e s as t h e r e a r e d i t r i g i o n a l h o l e s . Comparison o f

the bands a t f r e q u e n c i e s

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F i g u r e 2 . I n f r a r e d a b s o r p t i o n s p e c t r a o f the 8.4A h y d r a t e ( A ) , t h e 8.6A h y d r a t e ( B ) , t h e 10A h y d r a t e ( C ) , and the o r i g i n a l k a o l i n i t e used t o s y n t h e s i z e the t h r e e h y d r a t e s ( D ) .

American Chemical Society Library 1155 16th St., N.W. Washington, D.C. 20036 In Geochemical Processes at Mineral Surfaces; Davis, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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10A h y d r a t e spectrum w i t h t h e o t h e r two h y d r a t e s shows t h a t t h e g e n e r a l f e a t u r e s a r e s i m i l a r ; the 10A h y d r a t e has two bands a t 3585 and 3548 cm" from h o l e w a t e r v i b r a t i o n modes, and the h i g h f r e q u e n ­ cy bands above 3620 cm" have f e a t u r e s which a r e g r o s s l y s i m i l a r t o t h o s e i n the 8.6A h y d r a t e . The s i m i l a r i t y i n bands between 3600 and 3500 cm f o r the 10A and 8.6A h y d r a t e s i n d i c a t e s the e x i s t e n c e o f h o l e water i n the f o r m e r . Below 3500 cm" , the 10A h y d r a t e , i n common w i t h h y d r a t e d h a l l o y s i t e s , shows a v e r y broad a b s o r p t i o n c e n ­ t e r e d a p p r o x i m a t e l y a t 3250 c m . T h i s f e a t u r e i s a b s e n t from t h e two o t h e r h y d r a t e s and from t h e k a o l i n i t e . This absorption f a l l s i n t h e range o f l i q u i d water and has been a s s i g n e d t o t h e e x t r a water i n t r o d u c e d t o expand the h y d r a t e d c l a y from 8 . 4 / 8 . 6 A t o 10A. Thus, the i n f r a r e d s p e c t r a a l o n g w i t h the o b s e r v e d i n c r e a s e i n l a y e r t h i c k n e s s from r o u g h l y 8.4 t o 10 A, and t h e d e h y d r a t i o n d a t a (5., 27) i n d i c a t e t h a t when a d i h y d r a t e i s formed, h a l f t h e water ( h o l e water) i s a t t a c h e d t o t h e d i t r i g o n a l h o l e s o f t h e s i l i c a t e s u r f a c e w h i l e the o t h e r h a l f o c c u p i e s o t h e r s i t e s a t a g r e a t e r d i s t a n c e from the s i l i c a t e s u r f a c e . Lackin h y d r a t e , we c a n n o t d i r e c t l l a y e r o f water m o l e c u l e s . We do know t h a t they must occupy s i t e s o t h e r t h a n the d i t r i g o n a l h o l e s because t h e s e a r e c o m p l e t e l y f i l l e d by h o l e water ( £ ) . To d i s t i n g u i s h t h e two t y p e s o f w a t e r , the n o n - h o l e water i s termed " a s s o c i a t e d w a t e r " . Heat C a p a c i t y Measurements and I n t e r l a v e r Water S t r u c t u r e . The h e a t c a p a c i t y o f the i n t e r l a y e r water has been measured f o r the 10A, 8 . 6 A , and 8.4A h y d r a t e s between 110 and 275 Κ (24) and p r o v i d e s an i m p o r t a n t c l u e i n d e t e r m i n i n g t h e s t r u c t u r e o f t h e a s s o c i a t e d water molecules. A t a l l t e m p e r a t u r e s , the Cp f o r the i n t e r l a y e r w a t e r i n the 8.4A and 8.6A h y d r a t e s was n o t s i g n i f i c a n t l y d i f f e r e n t from published values for i c e . The water i n t h e 10A h y d r a t e d e v i a t e d from the i c e v a l u e s a t r o u g h l y 160 Κ and t h e d e v i a t i o n i n c r e a s e d as the temperature r o s e . A s h a r p peak i n t h e Cp began a t r o u g h l y 240 Κ and ended near the m e l t i n g p o i n t o f i c e ( F i g u r e 3 ) . The i n i t i a l d e p a r t u r e o f t h e Cp from i c e v a l u e s c o i n c i d e d w i t h an i n c r e a s e i n t h e p r o t o n NMR s i g n a l (25.). T h i s b e h a v i o r s u g g e s t s t h a t a t v e r y low t e m p e r a t u r e s , the water m o l e c u l e s occupy r e l a t i v e l y s t a t i c p o s i t i o n s i n the i n t e r l a y e r r e g i o n . The i c e - l i k e Cp o f t h e h o l e water t h r o u g h o u t the t e m p e r a t u r e range i n v e s t i g a t e d i s i n agreement w i t h the s u p p o s i t i o n d e s c r i b e d e a r l i e r t h a t the hole water i s r e l a t i v e l y s t r o n g l y bonded t o t h e d i t r i g o n a l h o l e s and r e m a i n s f i x e d . The peak i n t h e Cp t h e n must i n v o l v e p r i m a r i l y the a s s o c i a t e d water m o l e ­ cules. Examination o f a p r o j e c t i o n o f the t e t r a h e d r a l s u r f a c e o f the k a o l i n l a y e r shows a p s e u d o - h e x a g o n a l arrangement o f oxygen atoms (Figure 4). When a l l the d i t r i g o n a l s i t e s a r e o c c u p i e d by h o l e water (open c i r c l e s i n t h e f i g u r e ) , i t c a n be s e e n t h a t t h e y a r e approximately 5 A apart (Figure 4A). Hence, t h e r e i s no p o s s i b i l i t y o f hydrogen b o n d i n g between h o l e water m o l e c u l e s i n e i t h e r the 8.4A and 8.6A h y d r a t e s . The a s s o c i a t e d w a t e r m o l e c u l e s ( f i l l e d c i r c l e s i n the f i g u r e ) c a n be added i n two d i f f e r e n t o r d e r e d a r r a n g e m e n t s , one o f which i s shown i n F i g u r e 4. I n e i t h e r arrangement, t h e d i s ­ t a n c e between a s s o c i a t e d and h o l e water m o l e c u l e s i s on t h e o r d e r o f 3 A, a r e a s o n a b l e d i s t a n c e f o r hydrogen b o n d i n g t o o c c u r between h o l e water and a s s o c i a t e d w a t e r . These hydrogen bonds from a s s o c i -

In Geochemical Processes at Mineral Surfaces; Davis, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

3.

GIESE A N D COSTANZO

Water on the Surface of Kaolin

49

Minerals

40 ι

30 Η

LU _l Ο Έ

20 Η

WATER

< < LU ϋ 10

ICE

0Ί

.

0

100

200

·

.

300

400

TEMPERATURE (Κ) Figure

3.

between values of

The heat

the layers for

of

water

i n Reference

(Cp)

kaolinite

ice and l i q u i d

the intercalated

described

capacity

water

for

the water

intercalated

i n t h e 10A h y d r a t e . are also

was measured

shown.

using

the

Standard

The heat

capacity

procedure

2.

F i g u r e 4. A schematic r e p r e s e n t a t i o n o f the t e t r a h e d r a l surface o f k a o l i n i t e ( t r i a n g l e s ) showing t h e p o s i t i o n o f t h e h o l e water m o l e c u l e s (open c i r c l e s ) k e y i n g i n t o t h e d i t r i g o n a l h o l e s . The a s s o c i a t e d water ( f i l l e d c i r c l e s i n A) m o l e c u l e s a r e a r r a n g e d i n an o r d e r e d p a t t e r n which e x i s t s a t low t e m p e r a t u r e s . Disorder i n the a s s o c i a t e d water ( f i l l e d c i r c l e s i n B) i s c r e a t e d by i n c r e a s ­ i n g the temperature.

In Geochemical Processes at Mineral Surfaces; Davis, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

50

G E O C H E M I C A L PROCESSES AT M I N E R A L

SURFACES

a t e d water t o h o l e water a r e s t r o n g e r t h a n the bonds from the h o l e water t o the d i t r i g o n a l oxygens as shown by the lower i n f r a r e d a b s o r p t i o n f r e q u e n c i e s o f the former. As the t e m p e r a t u r e r i s e s , t h e model s u g g e s t s t h a t a few a s s o c i a t e d water m o l e c u l e s become m o b i l e . Those which have s u f f i c i e n t k i n e t i c energy c a n jump t o the v a c a n t s i t e s o f the a l t e r n a t e c o n f i g u r a t i o n (as i n F i g u r e 4 B ) . This d i s ­ r u p t s the o r d e r l y hydrogen bond scheme w h i c h e x i s t s a t l o w e r t e m p e r ­ a t u r e s and makes i t e a s i e r f o r o t h e r a s s o c i a t e d m o l e c u l e s t o jump. As the t e m p e r a t u r e c o n t i n u e s t o r i s e , t h i s jumping between the two c o n f i g u r a t i o n s , s i m i l a r t o m e l t i n g , l e a d s t o the peak i n Cp w i t h a maximum a t about 270 Κ ( F i g u r e 3 ) . I n summary, the model s u g g e s t s t h a t the water i n d i r e c t c o n t a c t w i t h the m i n e r a l s u r f a c e ( h o l e water) i s s t r o n g l y bonded t o t h e s i l i c a t e l a y e r . The second l a y e r o f water ( a s s o c i a t e d water) behaves v e r y d i f f e r e n t l y because i t has few i f any hydrogen bonds d i r e c t l y t o the s i l i c a t e l a y e r . A p p l i c a t i o n o f R e s u l t s to o t h e r S i l i c a t e M i n e r a l s Our model f o r the a d s o r p t i o a system w i t h few i f any i n t e r l a y e r c a t i o n s . However, i t s t r o n g l y r e s e m b l e s the model proposed by Mamy (13) f o r s m e c t i t e s w i t h monovalent i n t e r l a y e r c a t i o n s . The p r e s e n c e o f d i v a l e n t i n t e r ­ l a y e r c a t i o n s , as shown by s t u d i e s o f s m e c t i t e s and v e r m i c u l i t e s , should r e s u l t i n a strong s t r u c t u r i n g of t h e i r primary h y d r a t i o n s p h e r e and p r o b a b l y the next n e a r e s t n e i g h b o r water m o l e c u l e s as well. I f t h e c o n c e n t r a t i o n o f the d i v a l e n t c a t i o n s i s low, t h e n t h e water i n i n t e r l a y e r space between t h e d i v a l e n t c a t i o n s w i l l c o r r e ­ spond t o the p r e s e n t model. On the o t h e r h a n d , i f t h e c o n c e n t r a t i o n o f d i v a l e n t c a t i o n s approaches the number o f d i t r i g o n a l s i t e s , t h i s model w i l l n o t be a p p l i c a b l e . Such a s i t u a t i o n would o n l y be found i n concentrated e l e c t r o l y t e s o l u t i o n s . I n d i s c u s s i n g the a p p l i c a b i l i t y o f t h e p r e s e n t model t o s i l i ­ c a t e m i n e r a l s i n g e n e r a l , t h e r e a r e two c o n s i d e r a t i o n s : t o what e x t e n t do the exposed s u r f a c e s o f a g i v e n s i l i c a t e m i n e r a l mimic t h e d i t r i g o n a l h o l e s o f the c l a y m i n e r a l s , a n d , d u r i n g c h e m i c a l w e a t h e r ­ i n g , i s the s i l i c a t e m i n e r a l d i r e c t l y exposed t o the aqueous phase o r i s t h e r e an i n t e r v e n i n g phase o f d i f f e r e n t s t r u c t u r e and c o m p o s i ­ tion? The f i r s t p o i n t i s f a i r l y e a s i l y d e t e r m i n e d by i n s p e c t i o n o f t h e c r y s t a l s t r u c t u r e s o f the major s i l i c a t e g r o u p s . Nesosilicates ( i s o l a t e d t e t r a h e d r a ) and t h e s o r o s i l i c a t e s ( d o u b l e t e t r a h e d r a ) have l i t t l e resemblance t o the s t r u c t u r e o f t h e c l a y m i n e r a l s , t h e i n o s i l i c a t e s ( s i n g l e and d o u b l e c h a i n s ) which have c o r n e r shared t e t r a ­ h e d r a a r e s i m i l a r , p a r t i c u l a r l y the d o u b l e c h a i n s , p h y l l o s i l i c a t e s ( s h e e t s t r u c t u r e s ) c l e a r l y a r e s i m i l a r , and the t e k t o s i l i c a t e s may o r may n o t be s i m i l a r , depending on the e x t e r n a l s u r f a c e s exposed t o the aqueous phase. To the e x t e n t t h a t t h e m i n e r a l s a r e s i m i l a r , one would e x p e c t the model t o a p p l y . I t i s w e l l known t h a t d i s s o l u t i o n o f s i l i c a t e s , p a r t i c u l a r l y the c o m p o s i t i o n a l l y complex ones s u c h as t h e f e l d s p a r s , i s o f t e n i n c o n g r u e n t and t h a t p r e c i p i t a t e d p r o d u c t may c o a t the m i n e r a l s u r f a c e s . These c o a t i n g s may be ( c o m p o s i t i o n a l l y or s t r u c t u r a l l y ) s i m i l a r to c l a y minerals or r e l a t e d l a y e r s t r u c t u r e s ( k a o l i n i t e , s m e c t i t e , b o e h m i t e ) , and under t h e s e c o n d i ­ t i o n s , t h e model f o r adsorbed water may a l s o be v e r y g o o d . Little i s known i n d e t a i l about t h e amorphous phases which form a t t h e i n t e r f a c e between s i l i c a t e m i n e r a l s and the aqueous phase, and the

In Geochemical Processes at Mineral Surfaces; Davis, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

3.

GIESE A N D COSTANZO

Water on the Surface of Kaolin

model must be used w i t h c a u t i o n f o r cates.

Minerals

m i n e r a l s other than

51

phyllosili-

Summary An u n d e r s t a n d i n g o f much o f aqueous g e o c h e m i s t r y r e q u i r e s an a c c u r a t e d e s c r i p t i o n o f the w a t e r - m i n e r a l i n t e r f a c e . Water m o l e c u l e s i n c o n t a c t w i t h , o r c l o s e t o , the s i l i c a t e s u r f a c e a r e i n a d i f f e r e n t environment t h a n m o l e c u l e s i n b u l k w a t e r , and i t i s g e n e r a l l y agreed t h a t t h e s e adsorbed water m o l e c u l e s have d i f f e r e n t p r o p e r t i e s than bulk water. Because t h i s i n t e r f a c i a l c o n t a c t i s so i m p o r t a n t , t h e adsorbed water has been e x t e n s i v e l y s t u d i e d . S p e c i f i c a l l y , two major q u e s t i o n s have been examined: 1) how do the p r o p e r t i e s o f s u r f a c e adsorbed water d i f f e r from b u l k w a t e r , and 2) t o what d i s t a n c e i s water p e r t u r b e d by the s i l i c a t e s u r f a c e ? These a r e d i f f i c u l t q u e s t i o n s t o answer because the i n t e r f a c i a l r e g i o n n o r m a l l y i s a v e r y s m a l l p o r t i o n o f the w a t e r - m i n e r a l s y s t e m . To i n c r e a s e the proportion of surface t t h e i r large s p e c i f i c surfac mental m a t e r i a l s . Based on t h e s t u d y o f e x p a n d i n g c l a y m i n e r a l s , two models o f water adsorbed on s i l i c a t e s u r f a c e s have been p r o p o s e d . One s t a t e s t h a t o n l y a few l a y e r s (